What app is this icon with a square and a black triangle?

macos applications

Solution 1:

That is the icon for AirPlay, by Apple, and that menu bar item allows you to control the streaming/mirroring of content on your Mac to an Apple TV or other AirPlay server.

Turning AirPlay Mirroring on and off

If your Mac supports using your Apple TV as a display, an AirPlay menu appears in the menu bar when an Apple TV is on the same network. The AirPlay Mirroring menu icon turns blue when AirPlay Mirroring is on ().

Source: About AirPlay Mirroring in OS X

Related

Is it acceptable that 'fuel' is pronounced as 'fju:ə'
Is "pipe dream" an Americanism?
What does "third leg of the stool" mean?
Do "normal people" know the terms URL and GUI?
Why is "X on steroids" good?
What does the phrase “it’s like Groundhog Day every day” mean, and where does it originate?
Opposite of 'contaminate'
What does “it’s not going to suck itself ” mean?
Why does "one-night stand" mean sex?
Alternative, more commonly accepted parlance for "currentness"
What's the opposite of a greeting? [duplicate]
A word to describe setting-up (or preparing) the basic attributes of something