diff --git a/src/main/java/com/mythlane/gravityflip/command/GravityFlipCommand.java b/src/main/java/com/mythlane/gravityflip/command/GravityFlipCommand.java
index e9f432e..5fe3729 100644
--- a/src/main/java/com/mythlane/gravityflip/command/GravityFlipCommand.java
+++ b/src/main/java/com/mythlane/gravityflip/command/GravityFlipCommand.java
@@ -21,7 +21,7 @@ public final class GravityFlipCommand extends AbstractCommandCollection {
     public GravityFlipCommand(GravityFlipPlugin plugin) {
         super("gravityflip", "Commandes de gestion des zones Gravity Flip");
         this.addSubCommand(new GravityFlipWandSubCommand());
-        // 04-03 will addSubCommand(new GravityFlipDefineSubCommand(plugin));
+        this.addSubCommand(new GravityFlipDefineSubCommand(plugin));
         // 04-04 will addSubCommand(new GravityFlipListSubCommand(plugin));
         //        addSubCommand(new GravityFlipDeleteSubCommand(plugin));
         //        addSubCommand(new GravityFlipToggleSubCommand(plugin));
diff --git a/src/main/java/com/mythlane/gravityflip/command/GravityFlipDefineSubCommand.java b/src/main/java/com/mythlane/gravityflip/command/GravityFlipDefineSubCommand.java
new file mode 100644
index 0000000..ab60040
--- /dev/null
+++ b/src/main/java/com/mythlane/gravityflip/command/GravityFlipDefineSubCommand.java
@@ -0,0 +1,116 @@
+package com.mythlane.gravityflip.command;
+
+import com.hypixel.hytale.component.Ref;
+import com.hypixel.hytale.component.Store;
+import com.hypixel.hytale.math.shape.Box;
+import com.hypixel.hytale.math.vector.Vector3d;
+import com.hypixel.hytale.server.core.Message;
+import com.hypixel.hytale.server.core.command.system.CommandContext;
+import com.hypixel.hytale.server.core.command.system.arguments.system.RequiredArg;
+import com.hypixel.hytale.server.core.command.system.arguments.types.ArgTypes;
+import com.hypixel.hytale.server.core.command.system.basecommands.AbstractPlayerCommand;
+import com.hypixel.hytale.server.core.universe.PlayerRef;
+import com.hypixel.hytale.server.core.universe.world.World;
+import com.hypixel.hytale.server.core.universe.world.storage.EntityStore;
+import com.mythlane.gravityflip.GravityFlipPlugin;
+import com.mythlane.gravityflip.region.GravityFlipRegion;
+import com.mythlane.gravityflip.wand.WandSelectionStore;
+
+import javax.annotation.Nonnull;
+import java.util.UUID;
+import java.util.logging.Level;
+
+/**
+ * {@code /gravityflip define <name>} — consume the caller's wand selection
+ * (pos1 + pos2) to create and persist a new {@link GravityFlipRegion}.
+ *
+ * <p>Flow :
+ * <ol>
+ *   <li>Validate {@code name} via {@link DefineValidation#isValidName}.</li>
+ *   <li>Read the caller's selection from {@link WandSelectionStore}. Bail out
+ *       if either pos1 or pos2 is unset (builder must click 2 blocks first).</li>
+ *   <li>Compute componentwise min/max so the builder can click in any order.
+ *       Inflate max by +1 per axis so the max block is INSIDE the AABB (a block
+ *       occupies the unit cube between {@code (x,y,z)} and {@code (x+1,y+1,z+1)};
+ *       without inflation, a player standing on the max block is OUT of the
+ *       region — see {@code DefineValidationTest#boxFromCorners_inflateMax_includesMaxBlock}).</li>
+ *   <li>{@code registry.add(region)} — throws {@link IllegalArgumentException}
+ *       on duplicate name (T-04-03-05 mitigation — synchronized mutationLock).</li>
+ *   <li>{@code configHolder.save().join()} forces durability before responding.
+ *       Cost : a few ms disk. Acceptable in an interactive command.</li>
+ *   <li>Clear the caller's selection — {@code define} consumes it, next define
+ *       requires re-clicking 2 blocks (avoid chained accidental defines).</li>
+ * </ol>
+ *
+ * <p>On save failure the region remains in-memory (registry is already updated).
+ * We report this truthfully rather than silently rolling back — operators can
+ * inspect logs and the region is still active until restart.
+ */
+public final class GravityFlipDefineSubCommand extends AbstractPlayerCommand {
+
+    private final GravityFlipPlugin plugin;
+
+    /** Required STRING arg; full validation applied via {@link DefineValidation#isValidName}. */
+    private final RequiredArg<String> nameArg =
+            this.withRequiredArg("name", "Nom de la région (a-z0-9_-, 1-32 chars)", ArgTypes.STRING);
+
+    public GravityFlipDefineSubCommand(GravityFlipPlugin plugin) {
+        super("define", "Créer une région Gravity Flip à partir de la sélection wand");
+        this.plugin = plugin;
+    }
+
+    @Override
+    protected void execute(@Nonnull CommandContext ctx,
+                           @Nonnull Store<EntityStore> store,
+                           @Nonnull Ref<EntityStore> ref,
+                           @Nonnull PlayerRef playerRef,
+                           @Nonnull World world) {
+        String name = nameArg.get(ctx);
+        if (!DefineValidation.isValidName(name)) {
+            ctx.sendMessage(Message.raw(
+                    "[gravityflip] Nom invalide — attendu [a-zA-Z0-9_-]{1,32}."));
+            return;
+        }
+
+        UUID uuid = playerRef.getUuid();
+        WandSelectionStore.Selection sel = plugin.wandSelections().get(uuid);
+        if (sel.pos1 == null || sel.pos2 == null) {
+            ctx.sendMessage(Message.raw(
+                    "[gravityflip] Sélection incomplète — left-click puis right-click un bloc avec le wand avant define."));
+            return;
+        }
+
+        int[] mn = DefineValidation.componentwiseMin(sel.pos1, sel.pos2);
+        int[] mx = DefineValidation.componentwiseMax(sel.pos1, sel.pos2);
+        // Inflate max by +1 per axis so the block at maxBlock is inside the AABB
+        // (see DefineValidationTest#boxFromCorners_inflateMax_includesMaxBlock).
+        Box box = new Box(
+                new Vector3d(mn[0], mn[1], mn[2]),
+                new Vector3d(mx[0] + 1.0, mx[1] + 1.0, mx[2] + 1.0));
+        GravityFlipRegion region = new GravityFlipRegion(name, box, true);
+
+        try {
+            plugin.regions().add(region);
+        } catch (IllegalArgumentException ex) {
+            ctx.sendMessage(Message.raw(
+                    "[gravityflip] Une région nommée '" + name + "' existe déjà."));
+            return;
+        }
+
+        try {
+            plugin.configHolder().save().join();
+        } catch (Throwable th) {
+            plugin.getLogger().at(Level.WARNING).withCause(th)
+                    .log("[define] save failed for region '" + name + "'");
+            ctx.sendMessage(Message.raw(
+                    "[gravityflip] Région créée (en mémoire) mais persistance échouée — voir logs."));
+            return;
+        }
+
+        plugin.wandSelections().clear(uuid);
+        ctx.sendMessage(Message.raw(
+                "[gravityflip] Région '" + name + "' créée : "
+                        + "(" + mn[0] + "," + mn[1] + "," + mn[2] + ") → "
+                        + "(" + (mx[0] + 1) + "," + (mx[1] + 1) + "," + (mx[2] + 1) + ")"));
+    }
+}