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.

According to Vitrubio, molding or ring that decorates the column in the base and the capital. Characteristic of the Ionic order, it appears in the Doric from Roman times.

Astragalos are also called baquetillas and are designated by the name of rosary when, instead of being isolated, they form a series of pieces of round or oblong shape. It is also called astragallo to the molding that runs on the upper edge of the steps.

On doors
An astragal is commonly used to seal between a pair of doors. The astragal closes the clearance gap created by bevels on one or both mating doors, and helps deaden sound. The vertical member (molding) attaches to a stile on one of a pair of either sliding or swinging doors, against which the other door seals when closed. Exterior astragals are kerfed for weatherstripping.

An astragal may also be known as a “meeting stile seal”. It is sometimes confused with the wooden trim that divides the panes of a multi-light window or door, known as a muntin.

Source From Wikipedia