Mark off means to indicate or separate a specific area or item. The antonyms for mark off would be words that mean to join or unify. Some of the antonyms for mark off include unite, connect, join, integrate, and combine. These words connote actions that bring things together, thus eliminating separations or divisions. On the other hand, words like segregate, divide, partition, and separate are synonyms of mark off as they imply creating spaces or breaking up larger entities into smaller parts. Depending on the context, it is essential to use the correct antonyms to avoid any confusion or misinterpretation of the intended meaning.