Singing '3rd above' or (maybe more likely) '3rd below' is a loose description, but a useful one! Sometimes the 3rd will need to be a 4th (or even a 2nd) to fit with the harmony. Or a note or two in unison may be appropriate. But '3rd above' will do as a general label.

Just to muddy the waters, I've heard the term 'singing seconds' used for the same thing. 'Seconds' doesn't refer to the interval of a 2nd, just to 'adding a secondary part'.

The Everly Brothers epitomized the '3rds above' style. This transcription demonstrates the expected intervals - mostly 3rds above unless it HAS to be a 4th, and sometimes a 6th (an 'inverted 3rd') - but now and then (if my ear got it right) they slip in a 'forbidden' pair of parallel 5ths!

Singing '3rd above' or (maybe more likely) '3rd below' is a loose description, but a useful one! Sometimes the 3rd will need to be a 4th (or even a 2nd) to fit with the harmony. Or the 3rd might invert to become a 6th. Or a note or two in unison may be appropriate. But '3rd above' will do as a general label.

Just to muddy the waters, I've heard the term 'singing seconds' used for the same thing. 'Seconds' doesn't refer to the interval of a 2nd, just to 'adding a secondary part'.

The Everly Brothers epitomized the '3rds above' style. This transcription demonstrates the expected intervals - mostly 3rds above unless it HAS to be a 4th, and sometimes a 6th (an 'inverted 3rd') - but now and then (if my ear got it right) they slip in a 'forbidden' pair of parallel 5ths!

(Yes, it surprised me too! Feel free to listen to the recording and disagree.)