The term 'RAIN UNIT' is a combination of two words, 'RAIN' and 'UNIT'. RAIN is a form of precipitation that falls from the sky in the form of water droplets. A UNIT is a measure of something, such as a measurement of length or volume. The crossword solutions for the term 'RAIN UNIT' are DROP, which is a four-letter word meaning to fall or descend quickly and lightly. Therefore, the term 'RAIN UNIT' can be defined as a measure of the amount of RAIN that falls from the sky.