Real Dictionary

collar
countable noun: The collar of a shirt or coat is the part which fits round the neck and is usually folded over.

collar in American English
the part of a garment that encircles the neck
to put a collar on
noun: the part of a shirt, coat, dress, blouse, etc., that encompasses the neckline of the garment and is sewn permanently to it, often so as to fold or roll over

collar in British English
noun: the part of a garment around the neck and shoulders, often detachable or folded over

collar in Mechanical Engineering
noun: A collar is a ring or tube that fits onto a shaft or rod, for example to prevent a part from coming off the end of a shaft.