A person or thing that enlivens; something that adds energy, excitement, or interest.
From 'enliven' plus the agent suffix '-er,' creating a noun for something that performs the enlivening action. This noun pattern is standard in English for describing tools or agents of action.
An 'enlivener' could be almost anything—a joke is an enlivener at a boring party, a sprinkle of spice is an enlivener of bland food, and a good friend is an enlivener of a dull day—showing how flexibly English lets us use word-building patterns to describe the spark-bringing qualities of things.
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