Based loop

A based loop in $(X,x_0)$ is a path $\alpha :[0,1]\to X$ whose two endpoints are $x_0$.

A based homotopy between based loops keeps both endpoints fixed at the base point throughout the deformation.

The reverse of a path $\alpha$ is $\bar{\alpha }(t)=\alpha (1-t)$. Concatenating a path with its reverse is homotopic relative to endpoints to the constant path.