What classification of triangle is defined by having all angles measure less than 90 degrees?

A triangle with all angles measuring less than 90 degrees is classified as an acute triangle. The defining characteristic of an acute triangle is that each of its three interior angles is less than 90 degrees. This property establishes that the triangle does not contain any right angles or obtuse angles, making it distinctly acute.

In contrast, an obtuse triangle has one angle that measures more than 90 degrees, and a right triangle has one angle that measures exactly 90 degrees. A scalene triangle refers to a triangle where all sides have different lengths, but it can be classified as either acute, obtuse, or right, depending on the specific angle measurements. Since the question specifically asks about the angles, the classification of acute triangle is the most accurate in this context.