Plain English Meaning
The instruments on the panel that show how the aircraft is sitting in the air -- whether the nose is pointed up or down, and whether the wings are level or tilted to one side.
Full Definition
The flight instruments that display the aircraft's orientation in space relative to the horizon, primarily its pitch (nose up or down) and bank (wing tilt left or right). The attitude indicator is the central attitude instrument, but the heading indicator and turn coordinator also contribute attitude information.