Either of two crystalline isomers, C₁₀H₇CH₂COOH, obtained by replacing one of the hydrogen atoms of naphthalene by an acetic acid group; specifically (more fully "α-naphthaleneacetic acid") one which has the action of an auxin and is used to stimulate the rooting of plant cuttings, to initiate flowering of the pineapple, to prevent premature drop of fruit, and to improve the colour of apples.