Illustrated Glossary of Organic Chemistry

Terpenoid (isoprenoid): A molecule structurally similar to a terpene, except having a number of carbons atoms that is not a multiple of five (i.e., cannot be built exclusively from isoprene units), or whose skeleton has been modified in some way (such as oxidation).

Limonene is a terpene, derived from
biosynthetic combination of two isoprene units.
Menthol is a terpenoid. Its hydroxyl group
is not derived directly from isoprene; it must
be added by some oxidation process.