A wrestling hold in which one arm is passed under the opponent's arm from behind and the hand is applied to the neck (half nelson), or both arms and hands are applied (full nelson). In regular situations, this is a way to restrain someone from behind.
In sexual situations, the full nelson is incorporated as a form of reverse suspended congress, with the suspended partner's legs being restrained instead of their arms. Typically, this is a male partner suspending a female partner, then penetrating her anus with his penis in order to have anal sex, as the anus is easier for him to reach than the vagina.
Although the reverse suspended congress is the most common sexual use of the full nelson, it's possible for one man to have someone in a regular hold while another penetrates the restrained partner.
Try not to confuse this with the similar-looking missionary when the bottom character's legs are folded over the torso while the top character is grabbing their legs from the front. In a full nelson lift, the arms and hands should be drawn to show that the lifting person is behind the lifted person.