Sleet refers to two distinct forms of precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

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• Rain and snow mixed
  Rain and snow mixed
  Rain and snow mixed is precipitation composed of rain and partially melted snow. This precipitation can occur where the temperature in the lower part of the atmosphere is slightly above the freezing point...
  
  , snow that partially melts as it falls. (UK, Ireland and Canada)
• Ice pellets
  Ice pellets
  Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain...
  
  , one of three forms of precipitation in a US-style "wintry mix", the other two being snow
  Snow
  Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
  
   and freezing rain
  Freezing rain
  Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
  
  . (US)

The latter is referred to as sleet mainly by the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 National Weather Service

National Weather Service

The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...

. In some countries, such as Canada and the UK, sleet is not a recognized meteorological term. Environment Canada

Environment Canada

Environment Canada , legally incorporated as the Department of the Environment under the Department of the Environment Act Environment Canada (EC) (French: Environnement Canada), legally incorporated as the Department of the Environment under the Department of the Environment Act Environment...

will call it a mix of snow and rain or "a wintery mix" and will use the term "ice pellets" to describe frozen, granular precipitation.