To quit (close) a Mac app normally, choose Quit from the app's menu in the menu bar, or press Command (⌘)-Q. If the app doesn't quit, follow these steps to force the app to quit.

How to force an app to quit

Force Mac To Open App

Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu  in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.)

Then select the app in the Force Quit window and click Force Quit.

You can even force the Finder to quit, if it stops responding: Select Finder in the Force Quit window, then click Relaunch.

Learn more

Force Quit Mac App

How to force an app to close on your iPhone, iPad, or iPod touch.

Open Mac App Store

Control-click the app icon, then choose Open from the shortcut menu. Mac App Store – Only apps that came from the Mac App Store can open. Mac App Store and identified developers (default in OS X) – Only allow apps that came from the Mac App Store and developers using Gatekeeper can open.