(idiomatic) Attentive, active, busy or alert.

(snowboarding) A trick with one foot free from the bindings; instead it is placed beside the rear binding and boned far away from the board during airtime.

Having only a single eye, particularly when a greater number is normal. / Being able to see out of only one eye.

The highest class of motor racing sanctioned by the FIA.

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.