Saying or doing something impulsively in order to get rid of unwanted feelings, or to make the source of unwanted feelings stop whatever it is doing.
Sometimes also to punish the person who makes us feel something we don’t want to feel.
Can also be directed towards creating positive feelings that mask bad ones.
Basically, the concept acting out encapsulates all of the flavours and extremes of the temper tantrum.
The best analogy I can think of is food poisoning. “Food poisoning” happens when you eat something with dangerous bacteria that threatens to infect your stomach, gut, or the whole body. As soon as your body recognises the bacteria as a threat, it gets rid of it.
Sometimes this happens immediately and you vomit, but more often the stomach misses it and the bad bacteria makes its way into the intestines before it’s identified. And so it’s ejected downwards as diarrhoea, rather than upwards as vomit.
Acting out has a similar dynamic. An unwanted thought or sensation enters, and the mind identifies it as a threat, and then ejects it through either words or actions. Sometimes the reaction is immediate, and sometimes it is delayed. Sometimes it’s directed at the feeling’s source (the person by whom you feel abandoned, for example), and sometimes it’s redirected towards someone or something else.
Another good example is seen in movies and series. Any time anything even slightly emotionally difficult happens, the character immediately reaches for a bottle of alcohol, preferably whisky. Instead of containing and managing their feelings directly, they are overtaken by a reflex reaction which seeks out something to make the bad feeling go away.
Acting out actions include obvious things like shouting and tantrums, getting drunk, throwing things, hitting, biting, or scratching yourself or others. What you see more often are devious/sneaky actions like playing pranks, making deliberate mistakes, backstabbing/gossipping, and other forms of revenge.
However, acting out can also be a positive action like spontaneously giving pocket change to someone living on the street, or rescuing someone who has fallen/jumped onto the subway tracks.
In conversation, acting out can be seen in passive-aggression, micro-aggressions, abrupt changes of subject, lying, denial, gaslighting, and so on. Anything that fends of or deflects a criticism or an uncomfortable fact.