acute triangle (triangle all of the angles of which are acute)

a triangle (polygon)

a right-angled triangle

a tridecagon

obtuse triangle

(uranography) Winter Triangle - a bright asterism of the northern winter sky. Its corners are the stars Betelgeuse, Procyon, and Sirius.

(uranography) Summer Triangle - a bright asterism of the northern summer sky. Its corners are the stars Altair, Deneb, and Vega.

figure, diagram / graph / (geometry, mathematics) figure

(geometry) acute angle

(mathematics) equilateral triangle