diff --git a/crates/db/migrations/0004_field_definition.sql b/crates/db/migrations/0004_field_definition.sql
new file mode 100644
index 0000000..eee46d2
--- /dev/null
+++ b/crates/db/migrations/0004_field_definition.sql
@@ -0,0 +1,23 @@
+-- Registry of flexible field definitions (the "schema of schemas").
+CREATE TABLE field_definition (
+    id             UUID PRIMARY KEY,
+    key            TEXT NOT NULL UNIQUE CHECK (key <> ''),
+    data_type      TEXT NOT NULL CHECK (data_type IN
+                       ('text', 'localized_text', 'integer', 'date', 'boolean', 'term', 'authority')),
+    vocabulary_id  UUID REFERENCES vocabulary (id) ON DELETE RESTRICT,
+    authority_kind TEXT CHECK (authority_kind IN ('person', 'organisation', 'place')),
+    required       BOOLEAN NOT NULL DEFAULT false,
+    group_key      TEXT CHECK (group_key <> ''),
+    -- A term field must name a vocabulary; any other type must not.
+    CONSTRAINT term_has_vocabulary CHECK ((data_type = 'term') = (vocabulary_id IS NOT NULL)),
+    -- authority_kind is only meaningful for authority fields.
+    CONSTRAINT authority_kind_only_for_authority
+        CHECK (authority_kind IS NULL OR data_type = 'authority')
+);
+
+CREATE TABLE field_definition_label (
+    field_definition_id UUID NOT NULL REFERENCES field_definition (id) ON DELETE CASCADE,
+    lang                TEXT NOT NULL CHECK (lang <> ''),
+    label               TEXT NOT NULL CHECK (label <> ''),
+    PRIMARY KEY (field_definition_id, lang)
+);
diff --git a/crates/db/tests/migrate.rs b/crates/db/tests/migrate.rs
index 5588e37..680e992 100644
--- a/crates/db/tests/migrate.rs
+++ b/crates/db/tests/migrate.rs
@@ -53,3 +53,21 @@ async fn migrate_creates_vocabulary_and_authority_tables(pool: PgPool) {
         );
     }
 }
+
+#[sqlx::test]
+async fn migrate_creates_field_definition_tables(pool: PgPool) {
+    let db = Db::from_pool(pool);
+
+    for table in ["field_definition", "field_definition_label"] {
+        let regclass: Option<String> =
+            sqlx::query_scalar(&format!("SELECT to_regclass('public.{table}')::text"))
+                .fetch_one(db.pool())
+                .await
+                .unwrap();
+        assert_eq!(
+            regclass.as_deref(),
+            Some(table),
+            "table {table} should exist"
+        );
+    }
+}