A molding that is attached to one of a pair of swinging doors against which the other door strikes.
Merriam-Webster Online Dictionary
astragal (noun)1.
a narrow half-round molding
2.
a projecting strip on the edge of a folding door
astragal (Wikipedia)
Diagram of an astragalus as part of a Ionic order column
An astragal is a moulding profile composed of a half-round surface surrounded by two flat planes (fillets). An astragal is sometimes referred to as a miniature torus. It can be an architectural element used at the top or base of a column, but is also employed as a framing device on furniture and woodwork.
The word "astragal" comes from the Greek for "ankle-joint", αστράγαλος, astrágalos.