The definition of being hidden is being both unseen and unheard.
PHB Page 195, "If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses."
No, that's not the definition of hidden, that's simply specifying what subset of all the circumstances possible for hiding are always required to use the Unseen Attackers and Target rule. For example, you could hide and then begin movement from hiding. As you're doing your movement, you come out of darkness and through a patch of fog where your opponent can see you but not clearly, and then at the end of your movement you're back in darkness again. While in darkness, you're both hidden and able to use the Unseen Attackers and Targets rule, but during your movement through the patch of fog, although you remain hidden, you could not attack at advantage using that rule as you are seen just not clearly.
The side box on p. 177 of the PHB makes it clear that being seen or heard are only guidelines. The
errata for the PHB even specifically updates that language to "You can’t hide from a creature that can see you
clearly …”, adding the last word. Also if you listen to the Sage Advice video, posted above, Jeremy Crawford makes it clear that hiding takes into account any other criteria the DM believes relevant, and if you listen to this Sage Advice
podcast, the designers make it even more clear that they intend for the DM to decide what circumstances are appropriate for hiding, including all of the following:
- any specific traits or abilities
- whether the creature can see you
- whether the creature can hear you
- whether the creature can perceive your presence in any way
- whether the creature is distracted
- whether the creature is in a high state of alert or especially vigilant
- whether the creature is able to determine your position
- whether the creature perceives your presence as a potential attacker
- whether you are leaving any signs of your passage
So, being both unseen and unheard isn't the definition of Hiding under RAW. Gargoyles hide in plain sight, as can 10th level rangers. If you'd just been sprayed by a skunk and smell awful, the DM could decide you're not hidden. The best you can do to provide a definition of hiding is to approach it operationally ... which leads to our definition:
- Definition of being "Hidden" : Being "hidden" from an opposing creature means that you could attack that creature without them being able to perceive that your attack is coming.
That is wide enough to encompass all the other guidelines as well as this condition from the PHB p. 177:
"In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you.
However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen."
With that provision, it makes clear that being "seen" doesn't mean being in plain sight of a creature, and takes into account the creature's mental. Even an invisible creature that remains unheard may not be hidden. PHB p. 177:
"An invisible creature can't be seen, so it can always try to hide.
Signs of its passage might still be noticed, however, and it has to remain quiet".
Jeremy Crawford in the video I linked to previously mentions how an invisible creature jostling a table or causing footprints in the dust could be given away by "signs of its passage," for example.