The Collaborative International Dictionary of English v.0.48:

Delta \Del"ta\, n.; pl. Deltas. [Gr. de`lta, the name of the
   fourth letter of the Greek alphabet (the capital form of
   which is [Delta], Eng. D), from the Ph[oe]nician name of the
   corresponding letter. The Greeks called the alluvial deposit
   at the mouth of the Nile, from its shape, the Delta of the
   Nile.]
   1. The fourth letter of the Greek alphabet ([Delta] [delta]),
      corresponding to D. Hence, an object having the shape of
      the capital [Delta].
      [Webster 1913 Suppl.]

   2. A tract of land shaped like the letter delta ([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.
      [1913 Webster]

   3. (Elec.) 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.
      [Webster 1913 Suppl.]