Abstraction
Progressive simplification of a recognizable form into essential geometric shape, retaining what makes it identifiable while discarding surface detail.
In practice
Abstraction strips a recognizable form down step by step, keeping only what still lets people identify it. Designers do this constantly when making icons and pictograms: a realistic drawing of a bicycle becomes two circles, a triangular frame and a handlebar line, yet everyone still reads it as a bicycle. The skill lies in knowing which details carry the identity and which are surface noise that can go.
Often confused with Formalization
Abstraction simplifies a form toward its essential shape while keeping it recognizable; formalization simplifies a reference specifically to fit a medium's limits, such as cross-stitch or pixels.
Try this
Draw a cat five times, each with fewer lines, and stop at the last version a friend still recognizes.