A naked option is a short option position, meaning one the investor has sold rather than bought, that is not backed by the position that would satisfy the obligation if the option is exercised. The writer of a naked call does not own the underlying shares; the writer of a naked put does not hold a short stock position or an offsetting long put. In exchange for the premium received, the writer carries an open-ended obligation with nothing set aside against it.
The naming is worth one sentence, because the regulator uses both words. FINRA's rulebook calls this "uncovered." Rule 2360(b)(36) defines the term entirely in the negative, as a short option position that "is not covered", which sends the reader to Rule 2360(a)(10) for what covering requires. FINRA's own investor education calls it "naked." Its Options page writes of "an uncovered, or naked, call", and its glossary entries describe the writer of an uncovered call as the writer of what is "also known as a naked call" and the writer of an uncovered put as the writer of "a naked put". The two words name the same position, and this page leads with the one readers search for.