the fourth letter of the Greek alphabet (?, ?).

the consonant sound represented by this letter.

the fourth in a series of items.

anything triangular, like the Greek capital delta (?).

Mathematics . an incremental change in a variable, as ? or ?.

a nearly flat plain of alluvial deposit between diverging branches of the mouth of a river, often, though not necessarily, triangular

the Nile delta.

( usually initial capital letter ) a word used in communications to represent the letter D.

( initial capital letter ) Astronomy . a star that is usually the fourth brightest of a constellation

The fourth brightest star in the Southern Cross is Delta Crucis.