A shifter is someone or something that changes position, form, or function. In everyday life, you've likely seen a gear shifter in a car, the lever or knob that moves the transmission between park, reverse, and drive. Bicycles have shifters too, letting riders change gears to pedal easier uphill or faster on flat roads.

In fantasy stories, a shapeshifter is a creature that can transform its body into different animals or forms. Werewolves are shapeshifters that change from human to wolf form. Some myths feature gods or magical beings who shift into eagles, bears, or other creatures at will.

Workers on different shifts at a factory or hospital change places with other workers at different times of day. When you shift your weight from one foot to the other while standing, you're moving it. When someone shifts the conversation to a new topic, they're changing what everyone's talking about.

A shifter, then, is simply whatever or whoever does the shifting: the mechanism that changes gears, the worker who changes shifts, or the mythical being who transforms into something else entirely.