In light-frame construction, a trimmer is a timber or metal beam (joist) used to create an opening around a stairwell, skylight, chimney, and the like. Trimmers are installed parallel to the primary floor or ceiling joists and support headers, which run perpendicular to the primary joists.
It can also refer to a jack stud that supports a header above a window or door opening.
Traditionally, a stud which was less than full length was sometimes referred to as a cripple.
References
Trimmer (construction) Wikipedia(Text) CC BY-SA