A roundel in
heraldry is a small
disk used as a
charge on a
shield. In continental heraldry they are generally just termed roundels of such-and-such a colour, but in English heraldry special names are used for each one.
In the common colours:
A bezant is a roundel or (gold).
A plate is a roundel argent (silver).
A torteau is a roundel gules (red).
A hurt is a roundel azure (blue).
A pomeis is a roundel vert (green).
A pellet or ogress or gunstone is a roundel sable (black).
And with the rare colours:
A golpe is a roundel purpure (purple).
An orange is a roundel tenné (orange).
A guze is a roundel sanguine (maroon).
A roundel divided into a wavy white and blue representation of water is called a fountain. (In families called Sykes the fountain is termed a syke: one of those quaint prerogatives or affections that pop up in heraldry.)
A voided roundel (one with the centre cut out to leave only a ring) is termed an annulet.
Tiefling mentions that a torteau is also called a gout and a field semé (sewn or strewn) with them is called gouty. My source (Fox-Davies) doesn't mention this under roundels, but under semé discusses the goutte or drop; a field semé of gouttes is termed goutté or gutté and would look almost identical to a field semé of roundels. Once more, continental heraldry simply names the colour but British heraldry has special names: so goutté gules (red) is blazoned as goutté-de-sang (dropped with blood).