listener
n. countable A2 Elementary Oxford US //ˈɫɪsənɝ// UK //lˈɪsənɐ// lis·ten·er
n. a person who pays attention to what someone is saying or to music. In computing, it is also a part of a program that waits for a specific event or message to happen.
n. a person who attends to a speaker or a sound source. In a technical context, a function or object designed to wait for and respond to a specific event or signal within a software system.
She is a very good listener who always gives great advice.
The radio show has millions of regular listeners across the country every morning.
The developer implemented an event listener to trigger a specific animation whenever the user clicks the primary navigation button.
From listen + -er.
Usage
In technical contexts, often takes the preposition 'for' to indicate the event being monitored.