In the US “sleet” is the term for a winter precipitation that occurs when snow falls through a layer of warm air and melts into water droplets, then re-freezes into ice pellets as it passes through colder air closer to the ground. In many other areas that were part of the British empire that precipitation is called “ice pellets” and “sleet” instead refers to a mix of snow and rain. In the US that’s called a “wintry mix.”
Wintry mix in my part of costal New England generally refers to when the temp is fluctuating atound freezing, causing precip to come down as alternating / indeterminate area to area snow / sleet/ freezing rain. The worry being that the slushy mess will then freeze on the ground when the temperature drops.
For instance the radio station forecast yesterday was snow all day giving way to wintry mix from 9-11, then the temp dropping back to being snow 12-1 (when it cooled back down)
Which it did. We got like a foot of show, then it rained for an hour, then we got another hour of snow.