A nudo is both a hold and a category of holds. Nudos are knot holds, where the victim is as much entangled and trapped as they are stretched. Nudo holds are usually complex, and represent technical skill.
The hold most off considered as the primary nudo is also known as Nudo Misionero. The nudo hold was invented by Tarzan Lopez, and the "Misionero" name comes from El Signo (member of Los Missioneros De La Muerte), who's version of the hold is the one commonly considered a nudo.