In stock options, whats the difference between open interest and volume?

Question

In stock options, whats the difference between open interest and volume?

Answer 1

Open interest is the number of existing option contracts for that particular option.

Volume is how many option contracts have been sold/bought today.

Answer 2

Volume is the number of contracts traded on a particular day. The information is updated continously.

Open interest is the number of contracts that are open or outstanding. This information is updated once a day, at the end. For example, if someone, with no prior position, buys a call option and the person on the other side (seller) didn't have a prior position either, then the open interest increases by 1 contract. If either the buyer or the seller in the above case closes an exisiting position then the open interest doesn't change. Finally, if both the buyer and the seller close an existing position then the open interest will decrease by 1.