(1):

(n.) The fourth letter of the Greek alphabet (/ /), answering to D.

(2):

(n.) The closed figure produced by connecting three coils or circuits successively, end for end, esp. in a three-phase system; - often used attributively, as delta winding, delta connection (which see), etc.

(3):

(n.) an object having the shape of the capital /.

(4):

(n.) A tract of land shaped like the letter delta (/), especially when the land is alluvial and inclosed between two or more mouths of a river; as, the delta of the Ganges, of the Nile, or of the Mississippi.