What is "bromocresol purple"

n. (context organic compound English) A sulfonephthalein derived from a brominated derivative of cresol; it is used as an acid-base indicator

Bromocresol purple (BCP) or 5′,5″-dibromo-o-cresolsulfophthalein, is a dye of the triphenylmethane family ( triarylmethane dyes) and a pH indicator. It is colored yellow below pH 5.2, and violet above pH 6.8. In its cyclic sulfonate ester form, it has a pK value of 6.3, and is usually prepared as a 0.04% aqueous solution.