calotte

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A skullcap, especially one worn by Roman Catholic priests.

from The Century Dictionary.

• noun A plain skull-cap or coif of haircloth, satin, or other fabric, worn by the Roman Catholic clergy to cover the tonsure when exposed to drafts;
• noun in England, by serjeants-at-law on their wigs.
• noun In armor and costume, that part of any head-dress which covers closely the crown of the head: as, the calotte of the helmet.
• noun Anything having the form of a small cap, as the cap of a sword-hilt.
• noun In architecture, a dome or cupola, or something of similar form, as a cup-shaped ceiling, the head of an alcove, etc.
• noun In ornithology, a hood or cap of color upon the top of a bird's head.
• noun Also written calote and callot.
• noun In geology, an ice-cap or a glacier covering a large land area.
• noun In zoology: The pole of attachment of Dicyemidæ which are parasitic in the kidneys of certain cephalopods; the polar cap.
• noun In Polyzoa, a retractile disk formed at the aboral pole in the developing larva.

from the GNU version of the Collaborative International Dictionary of English.

• noun Such a cap, worn by English serjeants at law.
• noun Such a cap, worn by the French cavalry under their helmets.
• noun Such a cap, worn by the clergy of the Roman Catholic Church.
• noun to become a priest.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a skullcap worn by Roman Catholic priests
• noun archaic The vertical central area of the crown of a bird's head.
• noun architecture A round cavity or depression, in the form of a cup or cap, lathed and plastered; used to diminish the rise or elevation of a moderate chapel, alcove, etc. which would otherwise be too high for other pieces of the apartment.
• noun anatomy The upper (superior) or lower (inferior) half of the globe of the eye.