From یک (yek, “one”) + م (-om, “-th, -st (ordinal number suffix)”).
یکم • (yekom)
- (rare or archaic) first
- (in compounds) first
بیست و یکم ― bist o yekom ― twenty-first
اول (avval) is the standard word for "first" in modern Persian. Compound numbers, however, are always formed with یکم (yekom).