reservist

noun

military personnel in a reserve capacity (not employed on full-time active duty)

Wiktionary

noun

Etymology: Etymology tree English reserve Proto-Indo-European *-id- Proto-Indo-European *-yéti Proto-Indo-European *-idyéti Proto-Hellenic *-íďďō Ancient Greek -ῐ́ζω (-ĭ́zō) Proto-Hellenic *-tās Ancient Greek -τής (-tḗs) Ancient Greek -ῐστής (-ĭstḗs)bor. Latin -istader. Old French -istebor. Middle English -ist English -ist English reservist From reserve + -ist.

A soldier who is assigned as reserved: after training, no longer on full active duty.
“One of the men there, a reservist he said he was, told my brother he had seen the heliograph flickering in the west.”