naph·tha·lene·sulfonic acid \| ̷ ̷ ̷ ̷ˌ ̷ ̷+...-\noun Etymology: International Scientific Vocabulary naphthalene + sulfonic 1.: either of two crystalline monosulfonic acids C10H7SO3H obtained by sulfonation of naphthalene and used in the synthesis of dyes and naphthols 2.: any of numerous sulfonic acids derived from naphthalene