An Etymological Dictionary of Astronomy and Astrophysics

1. Evenly or uniformly arranged in space or time; orderly; well-ordered.
   Math.: Of a polygon, having all sides and angles equal.
   
2. Conforming to some accepted rule, discipline, etc.

Etymology (EN): M.E. reguler, from M.Fr., from
O.Fr. reguler, from L.L. regularis “continuing rules for guidance,” from L. regula “rule,” cognate with Pers. râst, → right.

Etymology (PE): 1) Bâsâmân, from bâ- “with, having” (→ hypo- + sâmân “order, arrangement, disposition; boundary, limit,” Lârestâni sâmon “sign or mark separating one field from another,” Gilaki, Tabari šalmân “a straight peace of wood or beam, post;”
Mid.Pers. sâmânak, sahmân “limit;” loaned into Arm. sahmân; cf. Skt. sīmān-, sīmā- “boundary, border, limit.”
2) Razan-mand, from razan, → rule, + -mand a possession suffix.