What is a terpenoid?

5-carbon chain---yes

oxygen--- No

with some double bonds--Yes

Terpenoids (or isoprenoids), a subclass of the prenyllipids (terpenes, prenylquinones, and sterols), represent the oldest group of small molecular products synthesized by plants and are probably the most widespread group of natural products.

Terpenoids can be described as modified terpenes, where methyl groups are moved or removed, or oxygen atoms added. Inversely, some authors use the term "terpenes" more .

They may be defined as a group of molecules whose structure is based on a various but definite number of isoprene units (methylbuta-1,3-diene, named hemiterpene, with 5 carbon atoms).

http://www.cyberlipid.org/simple/simp0004.htm

doctor answered it spot-on.

Terpenoids can be thought of as modified terpenes, wherein methyl groups have been moved or removed, or oxygen atoms added. (Some authors use the term "terpene" more broadly, to include the terpenoids.)

Terpenes are hydrocarbons resulting from the combination of several isoprene units.

Isoprene is a common synonym for the chemical compound 2-methylbuta-1,3-diene.