A variable resistor. A potentiometer used as a trimmer for occasional and small resistance adjustments. A Potentiometer is a variable resistor, while a trimmer is a variable resistor used for small circuit changes, so a trimmer potentiometer is a variable resistor used to make small changes in resistance.

In many cases a trimmer is also considered to be a variable resistor which was designed to only be used for occasional changes (before the wiper wears out). So in this case a trimmer potentiometer is a long life variable resistor being used only for small changes in circuit resistance. A trimmer potentiometer may be either a variable mechanical resistor or semiconductor potentiometer.