Definition of re-entry
Re-entry is a term used in poker tournaments. A re-entry tournament is similar to a rebuy tournament, but rather than having the ability to rebuy (in a rebuy tournament, you can buy in even when you still have chips), a re-entry tournament allows you to re-enter the same tournament when you are eliminated, but does not allow you to add chips when you still have chips. Some re-entry tournaments will allow you to re-enter only once, others will allow you to re-enter unlimited for the first hour of the tournament or the first 2 hours, depending on the structure. Be aware that a re-entry tournament allows you to re-enter for a limited time only.
Re-entry vs rebuy tournament
Generally, rebuy tournaments allow you to rebuy even when you still have chips. For example, if you lose your entire stack, you can rebuy a starting stack, but you can also rebuy a starting stack when you already have a starting stack (i.e. 2 stacks). These tournaments are more aggressive because many players will want to amass the most chips as quickly as possible. In re-entry tournaments, you get a second chance and you can't buy in when you still have chips. For the same buy-in, the re-entry tournament will have smaller payouts than the rebuy tournament.