2014 PHB (p. 292) has the definition of the condition, and references the Incapacitated condition (p. 291), with both entries in Appendix A: Conditions.
PHB24 (p. 376) also refers to the Incapacitated condition (p. 369), both are in the Glossary.
With respect to Incapacitated, it '24 extends '14’s inexcusably sparse definition to specify that the condition blocks Action, Bonus Action, and Reaction; breaks Concentration; prevents speaking, and imposes Disadvantage on Initiative rolls.
The change here is that the Stunned condition no longer affects a creature’s movement. which is an interesting step back from “no movement at all” - I would have expected some sort of slowing or impairing affect from the condition