Noun
Any one of three metameric substances, CH3.C6H4.OH, homologous with and resembling phenol. They are obtained from coal tar and wood tar, and are colorless, oily liquids or solids. [Called also cresylic acid.]
Source: Webster's dictionaryFor example, three isomers exist for cresol because the methyl group and the hydroxyl group can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). Source: Internet
Only one isomer of phenol or hydroxybenzene exists but cresol or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. Source: Internet