Imitation means copying the words, facial expressions, or actions of another person. Sometimes imitation is flattering, but often it's just annoying — like when your little brother does it to drive you crazy.
Use the adjective imitation to describe an object that pretends to be something else. Imitation paintings can be so real looking that it's hard to tell the fake from the genuine article. Some imitations have hung unnoticed on museum walls for years, only being exposed through the use of modern technology. A common proverb is "imitation is the sincerest form of flattery," although museum directors and people with obnoxious little brothers would probably disagree.