A triangle is a correction of five legs, each shorter than the last, with converging boundaries. The legs are labelled A, B, C, D, E, and each divides into three.
It is the only corrective form with five legs — which makes it easy to identify by count, and easy to confuse with an impulse, which also has five.
What is visible on the chart
A narrowing corridor: each next high lower than the last, each next low higher. Amplitude falls, the move fades.
The boundaries are drawn through the ends of the legs, and as they approach their meeting point less and less room is left — the form itself constrains where the last leg can end. That sets a triangle apart from a zigzag or a flat, where the place of completion is estimated from the proportions of neighbouring legs.
How it differs from an impulse
Both have five legs, which is what causes the confusion.
Two things separate them. The internal build: in an impulse legs 1, 3 and 5 divide into five, while in a triangle every leg divides into three. And the geometry: an impulse expands or holds its range, a triangle contracts.
Plus the overlap rule: in an impulse wave 4 does not enter wave 1's territory, whereas in a triangle the legs overlap constantly — that is its normal condition.
How it differs from the chart pattern
They are alike only on the surface. In classical technical analysis a triangle is a figure described by two lines, with a "target" after the breakout measured from the figure's height.
In wave markup a triangle is a corrective form with a specific number of legs and a specific internal build. No five legs, no triangle — however perfectly the picture narrows.
Where it occurs
Almost always in the second-to-last position: as wave 4 inside an impulse or as wave B inside a correction. More rarely as the final leg of a combination.
Hence a useful guide: a triangle usually means the containing move is not finished and this is a pause before the last leg. That is an observation about typical placement, not a rule: the markup gives no guarantee that such a continuation follows.
A common mistake
Declaring a triangle on three legs. Until there are five it is a hypothesis, and very often an unconfirmed one: the narrowing breaks on the fourth leg and the count has to be redone.