In Hold'em and Omaha a player who calls an unraised pot pre-flop is called a limper. They are said to have "limped in". Limping is generally considered to be a show of weakness.

The amount of a limp call will always be 1 x big blind, or half of that for the player in the small blind.

As to whether one can call the big blind a limper is undecided. While they did contribute only 1xBB, they had no choice in the matter. On the flop these players pose a worse problem than the limpers since they essentially have a random hand.

Uses

Though generally considered a weak play, there are some reasons why it would be considered.

A player with a premium hand in early-to-mid position may wish to simply call in order to get more pot contributors. Normally their hope would be for a late position raiser, in which case they would reraise to make their hand known. However, should no player bet and several limpers make it to the flop, this player will often find themselves in a tricky situation.

The other reason to limp would be in late position with a marginal hand. Should the pot odds be good enough it might make sense to take a cheap chance on the flop.