I believe many people are confused with "keysound" and "hitsound". To proceed the discussion before it become word tricks, let me introduce what they are.
Keysound
If you play games such as DJMAX or O2Jam or Beatmania IIDX, you must be extremely familiar with this.
Keysound, in full form "per-key sound effect", is a feature that many kind of music simulation game uses to enhance the play experience.
It allows players to directly play the whole song like what they do in real instruments.
A simple example would be, you see a note falling down, when you press the according button, a piano Do sound out.
If you don't press the button, no sound would be out.
So each note in the map is binded with a specific sound, when you play the map perfectly, you will hear the full music (played by yourself).
On the contracy, if you don't press the button at all (in the whole song), what you heard is only the background (usually complete silence).
Games like DJMAX, Beatmania IIDX, O2Jam use keysound.
Hitsound
Hitsound is a feature to provide feedback to players, in addition to the original music, to show whether a player have pressed correctly.
Like in other modes, hitsound is used to give audio hints on things like how you play the sliders.
Using the same example above, you see a note falling down, when you press the according button, the sound that it "presents" (in the background music) and an additional sound is presented.
If you don't, however the sound that it "presents" still sound, for it is sticked to the background music and is not possible to split out.
If you play the map perfectly, besides the Background music, a series of other sound is presented as a feedback.
On the contracy, if you don't press the button at all, you can only hear the background music flowing, without any feedback.
Osu! standard use hitsound.
Because of it is possible to impelement keysound using hitsound, the term "hitsound" is unclear in discussion. In the discussion, you may refer "fully customized hitsound" to keysound, that would be a good comparation.
Keysound
If you play games such as DJMAX or O2Jam or Beatmania IIDX, you must be extremely familiar with this.
Keysound, in full form "per-key sound effect", is a feature that many kind of music simulation game uses to enhance the play experience.
It allows players to directly play the whole song like what they do in real instruments.
A simple example would be, you see a note falling down, when you press the according button, a piano Do sound out.
If you don't press the button, no sound would be out.
So each note in the map is binded with a specific sound, when you play the map perfectly, you will hear the full music (played by yourself).
On the contracy, if you don't press the button at all (in the whole song), what you heard is only the background (usually complete silence).
Games like DJMAX, Beatmania IIDX, O2Jam use keysound.
Hitsound
Hitsound is a feature to provide feedback to players, in addition to the original music, to show whether a player have pressed correctly.
Like in other modes, hitsound is used to give audio hints on things like how you play the sliders.
Using the same example above, you see a note falling down, when you press the according button, the sound that it "presents" (in the background music) and an additional sound is presented.
If you don't, however the sound that it "presents" still sound, for it is sticked to the background music and is not possible to split out.
If you play the map perfectly, besides the Background music, a series of other sound is presented as a feedback.
On the contracy, if you don't press the button at all, you can only hear the background music flowing, without any feedback.
Osu! standard use hitsound.
Because of it is possible to impelement keysound using hitsound, the term "hitsound" is unclear in discussion. In the discussion, you may refer "fully customized hitsound" to keysound, that would be a good comparation.