diff --git a/internal/domain/instance/observation_target.go b/internal/domain/instance/observation_target.go
new file mode 100644
index 0000000..c77ac41
--- /dev/null
+++ b/internal/domain/instance/observation_target.go
@@ -0,0 +1,81 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package instance
+
+// Definition is the immutable effective configuration of an Instance.
+// Available extensions are observations, not part of the declared configuration.
+type Definition struct {
+	endpoint        Endpoint
+	adminCredential CredentialReference
+}
+
+func NewDefinition(endpoint Endpoint, adminCredential CredentialReference) (Definition, error) {
+	definition := Definition{endpoint: endpoint, adminCredential: adminCredential}
+	if err := definition.Validate(); err != nil {
+		return Definition{}, err
+	}
+	return definition, nil
+}
+
+func (d Definition) Endpoint() Endpoint                   { return d.endpoint }
+func (d Definition) AdminCredential() CredentialReference { return d.adminCredential }
+
+// Validate rejects invalid zero-value components even when constructors were bypassed.
+func (d Definition) Validate() error {
+	if err := d.endpoint.Validate(); err != nil {
+		return err
+	}
+	return d.adminCredential.Validate()
+}
+
+// ObservationTarget binds facts to a registration and its declared configuration.
+// It does not identify a physical PostgreSQL server or prove observation freshness.
+// Secret content refresh and same-target observation freshness remain application
+// responsibilities; no credentials or Secret contents are carried by this value.
+type ObservationTarget struct {
+	identity   Identity
+	revision   Revision
+	definition Definition
+}
+
+func NewObservationTarget(identity Identity, revision Revision, definition Definition) (ObservationTarget, error) {
+	target := ObservationTarget{identity: identity, revision: revision, definition: definition}
+	if err := target.Validate(); err != nil {
+		return ObservationTarget{}, err
+	}
+	return target, nil
+}
+
+func (t ObservationTarget) Identity() Identity     { return t.identity }
+func (t ObservationTarget) Revision() Revision     { return t.revision }
+func (t ObservationTarget) Definition() Definition { return t.definition }
+
+func (t ObservationTarget) Validate() error {
+	if err := t.identity.Validate(); err != nil {
+		return err
+	}
+	if err := t.revision.Validate(); err != nil {
+		return err
+	}
+	return t.definition.Validate()
+}
+
+// Matches rejects invalid targets before comparing values. Matching is necessary,
+// but not sufficient, for the aggregate to accept a fresh capability observation.
+func (t ObservationTarget) Matches(other ObservationTarget) bool {
+	return t.Validate() == nil && other.Validate() == nil && t == other
+}
diff --git a/internal/domain/instance/observation_target_test.go b/internal/domain/instance/observation_target_test.go
new file mode 100644
index 0000000..e40a38b
--- /dev/null
+++ b/internal/domain/instance/observation_target_test.go
@@ -0,0 +1,165 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package instance_test
+
+import (
+	"testing"
+
+	"git.ddupan.top/panxiao81/postgresql-tenant-operator/internal/domain/instance"
+)
+
+func targetParts(t *testing.T) (instance.Identity, instance.Revision, instance.Definition) {
+	t.Helper()
+	identity, err := instance.NewIdentity("uid-1", "shared")
+	if err != nil {
+		t.Fatal(err)
+	}
+	revision, err := instance.NewRevision(1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	endpoint, err := instance.NewEndpoint(validEndpoint())
+	if err != nil {
+		t.Fatal(err)
+	}
+	credential, err := instance.NewCredentialReference(validCredentialReference())
+	if err != nil {
+		t.Fatal(err)
+	}
+	definition, err := instance.NewDefinition(endpoint, credential)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return identity, revision, definition
+}
+
+func TestDefinitionRejectsInvalidComponents(t *testing.T) {
+	_, _, definition := targetParts(t)
+	cases := []struct {
+		endpoint   instance.Endpoint
+		credential instance.CredentialReference
+	}{
+		{instance.Endpoint{}, definition.AdminCredential()},
+		{definition.Endpoint(), instance.CredentialReference{}},
+	}
+	for _, tc := range cases {
+		value, err := instance.NewDefinition(tc.endpoint, tc.credential)
+		if err == nil || value != (instance.Definition{}) {
+			t.Fatal("invalid component accepted or partial value returned")
+		}
+	}
+	if err := (instance.Definition{}).Validate(); err == nil {
+		t.Fatal("zero definition accepted")
+	}
+}
+
+func TestObservationTargetRejectsInvalidComponents(t *testing.T) {
+	identity, revision, definition := targetParts(t)
+	cases := []struct {
+		identity   instance.Identity
+		revision   instance.Revision
+		definition instance.Definition
+	}{
+		{instance.Identity{}, revision, definition},
+		{identity, instance.Revision{}, definition},
+		{identity, revision, instance.Definition{}},
+	}
+	for _, tc := range cases {
+		value, err := instance.NewObservationTarget(tc.identity, tc.revision, tc.definition)
+		if err == nil || value != (instance.ObservationTarget{}) {
+			t.Fatal("invalid component accepted or partial target returned")
+		}
+	}
+	zero := instance.ObservationTarget{}
+	if err := zero.Validate(); err == nil {
+		t.Fatal("zero target accepted")
+	}
+	if zero.Matches(zero) {
+		t.Fatal("two invalid targets must not authorize observation reuse")
+	}
+}
+
+// Acceptance: docs/domain-instance.md §2/§6, observations cannot cross target bindings.
+func TestObservationTargetMatchesOnlySameBinding(t *testing.T) {
+	identity, revision, definition := targetParts(t)
+	original, err := instance.NewObservationTarget(identity, revision, definition)
+	if err != nil {
+		t.Fatal(err)
+	}
+	same, err := instance.NewObservationTarget(identity, revision, definition)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !original.Matches(same) || original.Identity() != identity ||
+		original.Revision() != revision || original.Definition() != definition {
+		t.Fatal("target did not preserve its declared binding")
+	}
+	changedIdentity, err := instance.NewIdentity("uid-2", identity.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	changedRevision, err := instance.NewRevision(2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, parts := range []struct {
+		identity instance.Identity
+		revision instance.Revision
+	}{{changedIdentity, revision}, {identity, changedRevision}} {
+		changed, err := instance.NewObservationTarget(parts.identity, parts.revision, definition)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if original.Matches(changed) || changed.Matches(original) {
+			t.Fatal("different registration or revision matched")
+		}
+	}
+	endpointValues := definition.Endpoint().Values()
+	endpointValues.Host = "other.example"
+	endpoint, err := instance.NewEndpoint(endpointValues)
+	if err != nil {
+		t.Fatal(err)
+	}
+	credentialValues := definition.AdminCredential().Values()
+	credentialValues.PasswordKey = "replacement"
+	credential, err := instance.NewCredentialReference(credentialValues)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, components := range []struct {
+		endpoint   instance.Endpoint
+		credential instance.CredentialReference
+	}{{endpoint, definition.AdminCredential()}, {definition.Endpoint(), credential}} {
+		changedDefinition, err := instance.NewDefinition(components.endpoint, components.credential)
+		if err != nil {
+			t.Fatal(err)
+		}
+		changed, err := instance.NewObservationTarget(identity, revision, changedDefinition)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if original.Matches(changed) {
+			t.Fatal("changed definition matched even with the same revision")
+		}
+	}
+	if original.Matches(instance.ObservationTarget{}) {
+		t.Fatal("valid target matched zero target")
+	}
+	if !original.Matches(same) {
+		t.Fatal("constructing changed targets mutated the original")
+	}
+}