A character with a mouth that is drawn wide--often shaped like a stretched-out "W" or a } rotated 90 degrees to the right--but which is usually only ever partially or slightly opened. This creates the visual impression of a snakelike mouth.
Also qualifying for this are especially wide :>-style closed mouths on a character who is already known for this sort of thing.
Notable examples of this are Ichimaru Gin, Orochimaru (Naruto), and occasionally Koko Hekmatyar (all of whom, unsurprisingly, have some connection to snakes)