heartwood, the central, woody core of a tree, no longer serving for the conduction of water and dissolved minerals; heartwood is usually denser and darker in color than the outer sapwood. Before the synthesis of aniline dyes, the heartwood of several tropical trees (sold collectively under the commercial name brazilwood) was used to produce blue, purple, and red dyes. As a tree becomes older, the heartwood increases in diameter, whereas the sapwood remains about the same thickness. See wood.

Dead, central wood of trees. Its cells usually contain tannins or other substances that make it dark and sometimes fragrant. Heartwood is mechanically strong, resistant to decay, and less easily penetrated by wood-preservative chemicals than other types of wood. One or more layers of living and functional sapwood cells are periodically converted to heartwood.

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See Wood#Heartwood and sapwood