荧 is a bound morpheme (b.f.) that appears in compounds relating to light and fluorescence, most commonly 荧光 (fluorescence, fluorescent). In classical texts it can also mean 'dazzled' or 'perplexed' (e.g., 荧惑, referring to the planet Mars or meaning 'to bewilder'), but this usage is rare in modern Chinese. You'll typically encounter it in technical and scientific terms.