It’s an accomplishment I wish I gained – one having done complete ????? for one’s past misdeeds. I generally try until Yom Kippur night, then it’s back to… So a BT is one that holds steadfast to his/her commitment to make a complete turnaround. Year-round!
Yes, the way the Rambam uses the term (since basically everyone was frum back then) is any Jew who commits a sin and then repents. Nowadays, it refers to frei hidden who come back to the fold.