Fun (verb, noun, more recently adjective). Used as an abstract noun, as in:

(a) anticipation of, or while engaged in an activity, to convince yourself or others, e.g. “hey, it’ll be fun!” “hey, we’re having fun!” by parents making the best of a wet day out, teachers seeking the love of their class during field trips, managers describing team-building exercises, new students during fresher’s week, members of office parties, stag and hen events.

(b) also used in the past tense, particularly in situations involving alcohol and substance abuse, or pronounced while leaving the: hospital, police station, realising you have no dry clothes, bedding etc. e.g. “that was fun!”

Facebook and other forms of social media have proved invaluable in ascertaining that fun was had via photographs and videos of group events. Other indicators of fun taking place include the use of multiple exclamation marks, e.g. “that really was fun!!!”