Astragal

A moulding applied to one stile of a French Door, Sliding French Door or French Casement window unit which the other door panel or window sash strikes. Usually head and footbolt devices will be found on the astragal side.

Astragal (Wikipedia)
Astragal architectural element as part of a Doric order column
Diagram of an astragalus profile as part of an 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.