(another long and tedious Yorkeman post follows …)
There’s obviously some mismatch in both use and understanding of the semantics here. What’s ‘correct’ can be less clear, as rather than being some absolute, meaning is often shaped by convention and usage, and this may be different in different environments.
A trivial example is that on a piano it’s not possible to play two notes of the same pitch, as @jjfagot points out, and yet on my guitar I can play an octave of C major scale with notes of the same pitch with 8 separate fingerings (without mixing shapes, which would make more) and I could play a ‘triad’ of three notes with the same pitch (if I had fingers 10 inches long).
Each note of said triad would not, however, sound the “same”. as the tonality would differ and they would have slight variations in tuning. I think it’s this sort of scenario which leads to different perceptions and usage … something is meaningless on a trumpet and an artistic expression on a guitar.
In @jjfagot’s example, it’s more like 0.002 +.0011 which is not 0.
So what we have is
@1stInversion → “two or more sounds or tones at the same pitch or in octaves.”
@jjfagot → “voices in unison to mean that they play exactly the same note”
@jamieh → “a single note and nothing else and should be called as such”
@ed66 → “unison therefore is two notes played at the same pitch”
I can see how there are different perceptions, maybe arising from different backgrounds @jjfagot cites an important caveat here " In classical music" where there are more or less no instruments which can play more than one given note of the same pitch (generally no guitars in a orchestra), whereas this type of tonality is more bread and butter to @1stInversion, and does actually have meaning.
I thought it would be useful (from my angle ) to pull the threads together of ‘Unison’ definitions like so
Noun words are often labels for concepts. It’s rather like two close together desert islands A and B having labelled ‘cat’ and ‘dog’ in opposite senses. So everyone on island A knows that dogs keep the mice at bay and on island B they know that dogs bark at night. All is well until they build a bridge between the islands, and meet, and confusion arises.
This board is that bridge, and we should treat these discourses as positive; we are all learning about other musical islands. Maybe we just need to remember that sometimes the meaning attached to words largely by convention is used differently by different tribes. Vive la difference …