Loss of skeletal muscle mass and function.
Usually associated with the natural aging process (i.e., becoming frail).
("Skeletal muscle" refers to voluntary muscles attached to the skeleton by tendons, rather than involuntary smooth muscles in organs, or cardiac muscles found in the heart.)
Studies imply that the average healthy person loses about 20% of skeletal muscle between the ages of 30 and 70. Muscle-strengthening exercise can ameliorate this loss.
Etymology: Greek, sarco, flesh + penia, poverty or loss. It's a fairly recent term, coined around 1989.
Here's an article from the National Center for Biotechnology Information.
Both Mom and Grandma used to help out the "old people" who had become frail, both while they were in their 80s, and Mom while she also had stage 4 cancer. I can only hope I inherited enough of any sarcopenia-defiant genes from their mix. Also...this word is motivating me to go do some more load-bearing yardwork. So, good.