Answer: A memory leak is memory that is allocated but never freed — usually from retain cycles. Over time it causes the app to use more and more memory and eventually crash.

How to find leaks with Instruments:

1. Xcode → Product → Profile (or Cmd+I)
2. Choose Leaks template
3. Use the app to trigger the suspected leak
4. Instruments marks leaked objects in red
5. Click a leak to see the allocation stack trace

Finding retain cycles with Memory Graph:

1. Run the app in Xcode
2. Click the Memory Graph button (3 circles icon in debug bar)
3. Look for objects that shouldn't exist — e.g. MyViewController that was dismissed
4. Click the object to see what's retaining it

Code Example — common leak:

// ❌ Retain cycle — both objects keep each other alive
class ViewController: UIViewController {
    var timer: Timer?

    override func viewDidLoad() {
        // Timer retains self strongly
        timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { _ in
            self.update()  // strong capture
        }
    }
    // timer never invalidated → ViewController never deallocated
}

// ✅ Fix
timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] _ in
    self?.update()
}

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    timer?.invalidate()
}