https://www.senzoku-online.jp/theory/classic/02/onmei-02.html - "シャープ、フラットの付いていない音を幹音（かんおん）といい、臨時記号（りんじきごう）や調号（ちょうごう）によって変化させられた音を派生音（はせいおん）といいます。"

OK. I was assuming that, say, the G#, C#, etc. in the key of A major were  幹音, but it seems they are 派生音
I'm wondering if a movement in A major modulated into A minor and used a natural accidental to bring the C# back to C natural, whether that C would be 幹音 or 派生音. I must ask a Japanese musician.

@@ -11,0 +12 @@
+<xref type="see" seq="2861234">幹音</xref>

https://yulala.org/theory/wb.php
「派生音」とは、幹音以外の音のことで、ピアノの黒鍵の出す音。

This is still wrong. As I pointed out below, the natural sign is an accidental. 派生音 does not refer to natural notes.

@@ -12 +11,0 @@
-<xref type="see" seq="1607490">臨時記号</xref>
@@ -14,3 +13,2 @@
-<gloss>raised or lowered note</gloss>
-<gloss>sharpened or flattened note</gloss>
-<gloss>note modified by an accidental</gloss>
+<gloss>sharp or flat note</gloss>
+<gloss>non-natural note</gloss>

Daijr too.
Discussions with my musician family members.

Yes, it's the note modified by an accidental; not the accidental itself.

@@ -11,0 +12 @@
+<xref type="see" seq="1607490">臨時記号</xref>
@@ -13,2 +14,3 @@
-<gloss>sharpened or flatted note</gloss>
-<gloss>non-natural note</gloss>
+<gloss>raised or lowered note</gloss>
+<gloss>sharpened or flattened note</gloss>
+<gloss>note modified by an accidental</gloss>

daijr: 音楽で，幹音を半音ずつ一回または二回音高変化させた音。変化記号をつけて表示される。
https://music.stackexchange.com/questions/17469/is-there-a-term-for-non-natural-notes

I think GG5 is wrong. The kokugos define 派生音 as a note that is raised/lowered a semitone (or tone) above/below natural pitch. Accidentals are notes that deviate from the key signature. This includes natural notes. (e.g. if the key signature contains a C sharp, then a C natural would be an accidental.)
It seems there isn't equivalent term for this in English.

@@ -13 +13,2 @@
-<gloss>accidental</gloss>
+<gloss>sharpened or flatted note</gloss>
+<gloss>non-natural note</gloss>

GG5, Daijr
派生音	1019