Fl.

Floruit (/ˈflɔərjᵿ.ɪt/ or /ˈflɒrᵿ.ɪt/), abbreviated fl. (or occasionally, flor.), in Latin meaning "he/she flourished", denotes a date or period during which a person was known to have been alive or active. In English, the word may also be used as a noun indicating the time when someone "flourished".

Floruit is the 3rd person singular perfect active of the Latin verb floreo "to bloom, flower", from the Latin noun flos, floris, "a flower".

Broadly, the term is employed in reference to the peak of activity for a person, movement, or such. More specifically, it often is used in genealogy and historical writing when a person's birth or death dates are unknown, but some other evidence exists that indicates when he or she was alive. For example, if there are wills attested by John Jones in 1204, and 1229, and a record of his marriage in 1197, a record concerning him might be written as, "John Jones (fl. 1197–1229)".

The term is often used in art history when dating the career of an artist. In this context, it specifically denotes the period of the individual's artistic activity, not just the known existence of the artist, which might differ significantly.

In some cases, it can be replaced by the words "active between (date) and (date)", depending on context and if space or style allows.

...
Wikipedia