Solo is a term used to describe the act of one player playing a mission in the Splinter Cell series. When playing solo, meaning without another human player, the player will only interact with NPCs (Non-Playable Characters) during the course of the mission or current game mode.
Although 'solo' can refer to the campaign portion, which describes the main single player campaign mode for the Splinter Cell series, solo typically means just playing alone. For the mode where two players play together in a single game session, see cooperative. For the mode(s) where players face each other, refer to multiplayer.
Solo mode was referenced as playing as being a "Lone Wolf" during Splinter Cell: Blacklist, where the game says that the player can play with a friend in cooperative instead of playing as a 'Lone Wolf'.