A Mental trait. From the Accelerant core rules:
"The effect is caused by some mental or emotional reaction. Abilities that work against Mental effects will work against effects with any of these traits. You may role play a Mental effect by crying out or stepping backwards as the effect is inflicted."
In the Madrigal campaign setting, the term has also been used to reference any noticably out-of-game conversation that occurs during gameplay. Since players are expected to always stay in character, references to out-of-game talk are said to be "speaking madness". Characters who speak too much madness during game may be repremanded by NPCs who seek to remove madness from the lands.