What is another word of saying triangle?

A 'TRIGON'.

'Tri' = '3'

'--gon' = a 2-dimensional figure. as in 'polygon'.

Trigon,

Tetragon =4, ( square, parallelogram etc.,)

Pentagon = 5

Hexagon = 6

Heptagon = 7

Octagon = 8

Nonagon = 9

Decagon = 10