Abbreviation: STM = "Stepper Motor" (Canon)

The abbreviations means "Stepper Motor"

Explanation from Canon

STM stands for Stepper Motor and is a applied to a new range of Canon lenses which feature a new design of focus motors which, along with a new iris mechanism are designed to eliminate (auditory) noise during video recording.

See examples of effected products

Canon EF 40mm f/2,8 STM