Eating together, according to Jewish law, means thanking God together, and a zimun is how you do it. One companion (in the literal sense of one with whom you break bread) invites the others to thank God or bensh together.
Zimun is the mishnaic Hebrew for invitation. In modern Hebrew, we say hazmanah. The root of both the ancient and current terms is the same: z’man, meaning time.
Zimun and hazmanah are natural outgrowths of z’man as the original, biblical meaning of the term was a set time — everything and every purpose under heaven has an appointed time (z’man) — based on Ecclesiastes 3:1. An invitation, or zimun, is an appointment; a temporal arrangement.
Indeed modern Hebrew does not distinguish between invitation and a summoning to appear. In Israeli restaurants, you mazmin your food; you also mazmin a taxi when you are ready to go home. In English, of course, we invite neither meals nor taxis, we order or book them. Ehud Olmert has been huzman for questioning by the police; now, he certainly was not invited, he was summoned. It is also interesting to note the similarity between the Hebrew z’man and the English summon.
A postprandial zimun is a ritual that acknowledges the meaningfulness of eating as a community — that eating together is part of being together.