Definition of limp
A limp is when you simply pay the big blind. On an blinds to 1/2 table, if you're in a UTG position, you'll have 3 options: fold and pay 0, limped (pay the big blind at 2) or raise (at 4 or more). It is possible to raise limped when no player has raised yet. If a player has already raised and you call, it will be called a cold call and not a limp. Generally, it is not advisable to raise a limp because the player who raises limp will not have the initiative and it could encourage other players to raise to limped. Thus, you a pot multiway with less equity in your hand.
If you make a limp, know why you are doing it.
If you know why you limped can be a good game. More and more world-class players are going to be using limp, especially one-on-one. When you do a limp, make sure you know why you're doing it and that it's the optimal option. For example, when playing one-on-one 2-player games, against some very aggressive opponents who 3bet a lot, the limp or limp-reraise option may be a good adjustment to make.