a usually soluble substance for staining or coloring e.g. fabrics or hair

a compound bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl.

a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)

an azo dye used as an acid-base indicator; used for titrations involving weak bases

a synthetic lemon yellow azo dye primarily used as a food coloring.