How is the term "affect" different from "emotion"? I have encountered a vast variety of definitions for affect, each of which contrasts it with the term "emotion" in a whole different way. Here are a few:


Affect denotes the visible/objective expression of an emotion.
Affect is the actual underlying subjective feeling an emotion.
Affect means the initial automatic reaction to a situation before cognitive appraisal helps build-up to the more complex emotions.


Also, given how practically (amongst professionals) both of these terms are often used interchangeably, I wonder if there's a clear non-controversial distinction between "affect" and "emotions".
There is a difference between affect and emotion, and the first two descriptions you quoted sum up the main way the two words are different


Affect denotes the visible/objective expression of an emotion.
Affect is the actual underlying subjective feeling [of] an emotion.


Wikipedia summarizes that in psychology, the word affect is used to describe the experience of feeling or emotion.  The word also refers sometimes to affect display, which is

a facial, vocal, or gestural behavior that serves as an indicator of affect (APA 2006).

Think of it in the sense that if you have an emotion in play, how do you describe how the emotion affects the person experiencing the emotion?
Emotions can interfere with how the person reacts to stimuli through valence (the &quot;goodness&quot; of the event , which is positive valence, or the &quot;badness&quot; of the event, which is negative valence — perceptional or actual), motivation or arousal (the state of being attentive, alert and awake). This is why affective disorders such as depression and anxiety disorders are not referred to as emotional disorders.
References
APA (2006). VandenBos, Gary R., ed. APA Dictionary of Psychology Washington, DC: American Psychological Association.