UHT milk, or Ultra High Temperature milk, is a type of milk that has been heated to a high temperature (around 135°C or 275°F) for a short period of time. This process kills harmful bacteria and extends the milk's shelf life, allowing it to be stored without refrigeration until opened. Once opened, UHT milk should be refrigerated and consumed within a few days, similar to regular milk. It is often packaged in aseptic containers, which help maintain its freshness. UHT milk is popular in many countries due to its convenience and long shelf life.