Lets say im playing a violin piece that's primarily in C major/G mixo-Lydian but i start to use notes outside of the scale without any chord accompaniment , technically those notes wouldn't be referred to as non chord tones, which include passing tones, incomplete neighboring tones,and etc. So what is the term for using out of scale notes on a monophonic instrument, and if possible can you add some more insight on the topic..Thanks!
It depends how the notes are being used. Technically they could be non-chord tones. You can specify chromatic with NCT's like chromatic appoggiatura or chromatic passing chord. Also, the chromatic tones could result in a shift in key. If it's temporary you could call it a tonicization, or if it is longer it could be called a modulation. Another term that could be appropriate is secondary dominant. It depends on the musical context. Even if the part is monophonic, if chords are being arpeggiated, then using some chord terms would be fine.