apex
top, tip
The word "tip" fits into that category nicely.
tag
An ell is an obsolete unit of measurement - the length of an arm from elbow to tip of the middle finger.
apex
The tip of a triangle is known as a vertex. A triangle has 3 vertices, 3 sides, and 3 angles. As well, apex can be described as the pointed end of a triangle tip.
tip
The vertex or the apex. I prefer apex to describe the upper most tip of a triangle.
top, tip
The word "tip" fits into that category nicely.
untip retip detip
The tri-tip roast, sometimes called a triangle roast, is the portion of meat that is at the bottom of sirloin.
I suppose you could re-tip something you have already tipped, but it's not pretty.
tag
The vertex
A notch