Oh, dude, that's easy. You call it a "solo piece" or a "solo composition." It's like when your friend decides to go solo in a boy band - they just want all the spotlight to themselves, you know? So yeah, it's basically a piece of music written for one instrument to show off its skills without any distractions.
A note.
It means someone individually plays a short piece of music (usually the leader), and the rest of his/her band replies to it by doing another short piece of music.
A short piece of music played in honor of a person or event is call a "Baroque".
The official term is "diegetic music", but it's quite academic and archaic. "Source music" is the more common term.
Simple ignorance, I guess. There's no real reason. But you're right, a song is the name of a piece with lyrics.
A note.
Ostinato
It means someone individually plays a short piece of music (usually the leader), and the rest of his/her band replies to it by doing another short piece of music.
double line
It was written by Wolfgang Amadeus Mozart, a composer in the Classsical Era. For this reason, you could call it a classical piece.
That's called a duet.
A short piece of music played in honor of a person or event is call a "Baroque".
It means someone individually plays a short piece of music (usually the leader), and the rest of his/her band replies to it by doing another short piece of music.
"Piano" in music means "softly", from the Italian. The instrument we call the piano has a full name of "pianoforte", meaning soft, loud. The name reflects the wide range of volume that the instrument has.
"Playing by ear" describes the ability of a musician to to reproduce a piece of music without having observed another musician playing the piece.
The Last Post is a British bugle call that is often played at military funerals and remembrance services. It is not a written piece of music attributed to any specific composer, as it is a traditional bugle call used in military ceremonies.
these could be anything, depending on personal opinion.