Straddle, the act of doubling the blinds.
Straddling the blinds was originally adding another blind double the size of the big blind. There are many reasons a player might do this, typically just to create a little extra action. Most public brick and mortar card rooms accommodate allowing players to straddle. Card rooms typically only allow straddling the blinds under the gun.
However variations have evolved. Straddling on the button, also known as a Mississippi straddle is becoming more common. Straddling from any position, also known as a sleeper bet is becoming more common.
Straddle and Sleeper bets from Wikipedia.