Transverse Comoving distance
Angular diameter distance is the reduced circumference of the circle, centered at our location, on which the object was located when it emitted the light (or the reduced area of the sphere if you prefer). If the universe is spatially flat then this is the same as the metric radius of the circle. In general it's related to the radius by the function $S_k$ defined here, where $k$ is the Gaussian curvature at that time.
Transverse comoving distance is the same thing scaled up by $1{+}z$, i.e., the reduced circumference of the circle on which the same object is located at the present cosmological time, if it had no net peculiar motion in the mean time. It's therefore equal to (radial) comoving distance if the universe is spatially flat.