Flux refers to flow.
Flux Density (or, commonly, Flux for short) is the amount of flow of something per unit area, in fluid dynamics, magnetism, Electromagnetic Radiation (EMR) and other fields.
Flux can also refer to total "flow" from something, such as EMR from a star, in the phrase Radiant Flux.
Extragalactic Background Light (EBL)
Poynting Vector (S)
Spectral Energy Distribution (SED)
Submillimeter Galaxy (SMG)
Spectral Index (α)
Young Stellar Object (YSO)