"Dw i mewn ymarfer."
Translation:I am in a practice.
What does "a practice" mean in this context? Like, "I'm in the middle of a practice session"?
Yes, exactly that.
eg 'Ble dych chi nawr? - Dw i mewn ymarfer.'
Where are you now? - I'm in a practice/training (session)'
The word session is not needed in Welsh but is implied.
"I'm practising" would be Dw i'n ymarfer, I believe -- using the verbnoun and this the particle yn, rather than the preposition mewn.