it's often a concept that people aspire to that gets in the way of who they really are.  this word is a red flag and reminds me of people who think "Zen" means you are always serene somehow or happy.  this is not possible.  most of the time we are neutral.  we can't be happy or serene if we don't experience the whole range of our feelings, our relative minds work by contrast.  practice is the deletion of concepts such as this and then the deletion of the deletion to the point that one forgets the self.  (as dogen says "to forget the self is to study the self".  serenity can arrive naturally but forcing it (like "those" Zen students on retreat who put on a Serenity mask while at the same time visibly look like they are jabbing a fork in their hand under the table.

i wrote this piece about emotions a while ago and touched on what i am referring to above  in it.

sacredpeak.ink/2023/10/19/the-strength-of-strings/

in terms of a definition, real serenity is a natural result of dropping our baggage.  the half smile of a person steeped in practice is this smile but that person has feelings and displays those as well exactly in accord with themself in each moment.  if you want to see a true half smile, DaVinci painted a very famous painting with one that people misread IMO.

@SGT.FISH thank you for asking this question.