Mental Formations is a phrase that one will see often in reading about Buddhism. It is derived from Saṅkhāra which is a Pali word, that is based on the Sanskrit word saṃskāra according to Wikipedia..
I like to think of it alternatively as thoughts, emotions, cravings, or feelings. Plum Village has a list of 51 different ones.
In meditation one gets a glimpse of the reality that mental formations come and go, in an endless stream. The Secular Buddhist Association has some particularly interesting thoughts on this topic. One also notices that they come and go, whether or not you fight against them. Just watching the thought seems to make it disappear. I suppose it’s because one can’t really multitask, just sort of multithread.
For the Theravada inclined Discourse.sutta central gives the sutta/sutra references to Saṅkhāra.
As always, there’s tons of references on the web related to this topic. Leave a comment to add yours.