Fourth letter of the
Greek alphabet, capital Δ and lower-case δ. The HTML codes for it are Δ and δ
It derives from the Phoenician letter for D, related to Hebrew ד daleth and Arabic د dal. The name means 'door', or more anciently, 'tent-flaps', whence its triangular shape. The Greeks also used it to name the triangular lands of the lower Nile, and we use it for any such river delta.
The letter was adopted into the Roman alphabet via Etruscan as D. In the Cyrillic alphabet the shape became Д.
In Ancient Greek it had the pronunciation of D in dot. In Modern Greek it developed a fricative pronunciation, a DH sound as in this, that, phonetic symbol [ð]. The original D sound also occurs in Modern Greek, but spelt differently: ancient ντ [nt] changed to [nd]. Then this came to be used to write D in foreign words: so Donald would be written as if Ntonalnt.