Is sleet freezing rain?

In the UK, Ireland, and Canada "sleet" refers to a mixture of rain and snow.

In the USA, "sleet" refers to ice pellets and is one of 3 forms of precipitation categorized as "wintery mix"; the other 2 forms are snow and freezing rain.

Sleet, by the UK definition, occurs when snow falls into a region just enough warmer to start melting the snow, but not all of it. Most of the time it gets just enough colder over the course of the storm to transition to straight up snow leaving a wet layer at the bottom. Occasionally it will instead warm up just enough to just barely melt all the snow on the way down and transition to freezing rain.

The ice pellets described in the US term occur when snow from high up falls through a region warm enough to melt it, then enters a lower zone where it is cold enough to freeze it again. If that lower zone almost refreezes it, then you get freezing rain instead.

In both cases sleet starts out as snow.

In practical terms, sleet is right on the borderline between freezing rain and snow; if it gets a little colder, you get just snow, if it gets just a tiny bit warmer, you get freezing rain which hits as a liquid but quickly freezes to a layer of ice on most surfaces.