decisive

Definitions:

Having the power or quality of deciding; final; conclusive; firm and resolute of purpose.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Final; conclusive; having the power to settle a contest or an event.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Final; conclusive; positive.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (adjective) (adj.) (adjective)
Decisiveness.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
Having the power of deciding: final: positive.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (adjective) (adj.) (adjective)
Final; conclusive; as, a decisive victory; prompt; positive; determined.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Decisively.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)

Usage examples:

" Action," he responded, " decisive and immediate!"
- In Direst Peril by David Christie Murray
Though the numbers engaged were insignificant, and though the victory was easily won, the battle of Chacabuco was decisive in the struggle between Spain and her revolted subjects in the southern colonies.
- The South American Republics Part I of II by Thomas C. Dawson
Who could avoid seeing its decisive significance for the age?
- A History of England Principally in the Seventeenth Century, Volume I (of 6) by Leopold von Ranke