A pantsuit is a matching set of clothes for a woman: trousers and a jacket cut from the same cloth, made to be worn together. It is the female counterpart of a man's suit, and it is business clothing — you wear it to an office, a meeting, a court, a conference, an interview. The word carries a little history with it. For a long time a woman at work was expected to wear a skirt, and trousers were a break with that; so the pantsuit came to stand for a woman doing serious professional work, and in American politics it became strongly associated with women in high office. That association is still alive, sometimes admiringly, sometimes as a joke. It is mainly an American word. British speakers usually say "trouser suit", because in British English "pants" means underwear — so "pantsuit" sounds odd or funny to a British ear. You will also see it written as two words, "pant suit", or with a hyphen; all three spellings are around, and "pantsuit" is now the most usual.