An Etymological Dictionary of Astronomy and Astrophysics

1. A small, hard seed of plants, especially the seed of cereals.
   

2. A tiny portion or particle of something such as sand or salt; → dust grain.

Etymology (EN): M.E. grain, grein, from O.Fr. grein, from L. granum “seed;” akin to corn.

Etymology (PE): Dâné “grain, seed;” Mid.Pers. dân, dânag “seed, corn;” Av. dānô- in dānô.karš- “carrying grains; an ant;” cf.
Skt. dhânâ- “corn, grain;” Tokharian B tāno “grain;” Lith. duona “corn, bread.”