Step 1: Recall how grains and boundaries behave differently with temperature.
At room temperature, grain boundaries are disordered regions that block dislocation motion effectively, so they tend to be stronger than the grain interiors, which is the basis of the Hall Petch strengthening idea. As temperature rises, atoms along the boundaries become mobile enough to slide and diffuse, so the boundaries gradually lose strength relative to the grains.
Step 2: Picture the crossover point.
Since grains are stronger relative to boundaries at high temperature but weaker at low temperature, there must be some specific temperature where the two strengths are exactly equal, this is the crossover between the two competing trends.
Step 3: Name that crossover.
This special temperature is called the equi-cohesive temperature, literally meaning equal cohesive strength, below it the grain boundaries dominate in strength, and above it the grains themselves become the weaker link, which explains why materials tend to fail along boundaries at high service temperatures such as during creep.
\[ \boxed{\text{equi-cohesive}} \]