Not certain; unsure. / Not known for certain; questionable. / Not yet determined; undecided. / Variable and subject to change. / Fitful or unsteady. / Unpredictable or capricious.

(with "the") Something uncertain.

Misspelling of uncertainty.

The quality of being uncertain.

(idiomatic) (of a statement, especially a threat or admonishment) Bluntly, forthrightly; explicitly, categorically.

(cricket) The area near the batsman's off stump where, if a ball passes that area, the batsman is unsure whether to play it (and risk an edge) or leave it (and risk being bowled).

(uncountable) Doubt; the condition of being uncertain or without conviction. / (countable) Something uncertain or ambiguous.

(quantum mechanics) The Heisenberg uncertainty principle.

(physics, quantum mechanics) The principle that there is an absolute limit on the combined accuracy of certain pairs of simultaneous, related measurements, especially that of the position and momentum of a particle. Originally posited as a problem of measurement, it was soon refined as an inherent property of the universe.

plural of uncertainty