An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.
a sight for sore eyes
a welcome sight
The man was a sight for sore eyes when he returned to work after a long holiday.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning other than its literal meaning of the words in it.