delta

n.
1 a triangular tract of deposited earth, alluvium, etc., at the mouth of a river, formed by its diverging outlets.
2 a the fourth letter of the Greek alphabet. b a fourth-class mark given for a piece of work or in an examination.
3 Astron. the fourth star in a constellation.
4 Math. an increment of a variable.
Phrases and idioms:
delta connection Electr. a triangular arrangement of three-phase windings with circuit wire from each angle. delta rays Physics rays of low penetrative power consisting of slow electrons ejected from an atom by the impact of ionizing radiation. delta rhythm (or wave) low-frequency electrical activity of the brain during sleep. delta wing the triangular swept-back wing of an aircraft.
Derivatives:
deltaic adj.
Etymology: ME f. Gk f. Phoen. daleth