(geometry) A triangle having a right angle as one of its interior angles.

(geometry) A triangle having at least two sides equal.

A situation in which two suitors vie or compete for the love of the same person.

(geometry) A triangle having all three sides equal.

(British spelling) A plane triangular graph on which are drawn all the colours that can be made by the additive mixing of red blue and green; these three primary colours are at the apices, while white is in the middle.

(American spelling) Alternative form of colour triangle

(mathematical analysis) The inequality that states that the magnitude of the sum of two vectors is less than or equal to the sum of the magnitudes of the vectors, or any equivalent inequality in other spaces.

(uranography) A bright asterism of the northern summer sky. Its corners are the three 1st-magnitude stars Altair (α Aql), Deneb (α Cyg), and Vega (α Lyr).

(informal) One of several denominations of triangular postage stamps issued by the Cape of Good Hope during the period 1853-1864.

[the W- T-] (uranography) A bright asterism of the northern winter sky. Its corners are the three 1st-magnitude stars Betelgeuse (α Ori), Procyon (α CMi), and Sirius (α CMa).