Unique, singular, sui generis.

A unique example.

(of a railway) having a single set of lines so that trains can run only in one direction at a time; single-track / limited to one idea or thought

The state of being one-sided

Needing only a single attempt to become effective. / Unique; occurring only once. / Performing all the necessary steps on every occurrence, rather than relying on previous setup.

(television, film, comics) A television program, film, or comic book that is not part of a series. / (film) A cinematographic shot of a person talking to camera; a talking head.

(video games) To kill or destroy with a single hit.

Misspelling of one-legged.

plural of hammer-on

(colloquial, figuratively) One's sanity; one's mental faculties.