Can you re-define 'Elizabethan Madrigals' or 'Medieval Hymns' or 'Victorian Parlour Ballads'?

Are those definitions? Or are they categories?

Roud explains what he means by 'folk song' for the purpose of his book. He seems to have made a determined effort to try to find out what 'the folk' sang without being constrained by other people's pigeonhole labels.