From The TimeSplitters Wiki

A Kill is when one player causes another player to die by depleting their health with weapons.

Points and Opposites

In most game modes a kill counts as a point, and a certain number of kills/points are required to win. The opposite of a kill is a Loss, which means the player was killed by another player. A loss usually makes the player lose a point, unless the scoring system is set to "Kills" only.

Effects

When one kills a player, that player suffers the effects of a loss. If you kill a Virus that virus spawns somewhere else, however for a short time their body still has the effects of being a virus. The same thing goes for Flame Tag. If you kill someone in an Elimination match, you make them loose a life. In Vampire, kills are required to stay alive, as they replenish your "blood".

One Shot Kill

A One Shot Kill is when a single projectile is 100% lethal. The best weapons for these are Explosives, although if you put the setting One Shot Kill to on, any weapon is a one shot kill weapon.