Synonyms for Decisive:

adj.

aggressive (adjective)
assertive.
all (adjective)
critical.
authoritative (adjective)
definitive.
certain (adjective)
absolute, assured, certain, clear, confident, convinced, decided, definite, doubtless, incontestable, incontrovertible, indisputable, indubitable, irrefutable, patent, positive, set, steadfast, sure, trusting, undeniable, undoubted, unequivocal, unmistakable, unquestionable, veritable, Irrefragable.
definite (adjective)
absolute, assured, certain, conclusive, critical, crucial, decided, definitive, determined, fateful, final, firm, forceful, intent, peremptory, positive, resolute, resolved, set, strong-minded.
final (adjective)
closing, concluding, conclusive, final, last.
positive (adjective)
positive.
resolute (adjective)
bold, bullheaded, committed, constant, courageous, dauntless, dedicated, deliberate, determined, devoted, diligent, dogged, earnest, faithful, fearless, firm, headstrong, hearty, implacable, indefatigable, indomitable, inexorable, insistent, intent, loyal, persevering, persistent, plucky, relentless, reliable, resolute, resolved, ruthless, serious, spunky, staunch, steady, steely, stern, strong-minded, strong-willed, stubborn, tenacious, thorough, tireless, tough, true, unbending, unchangeable, uncompromising, unflinching, unwavering, unyielding, valiant, willing, zealous, iron-willed, unshakeable, decided, set, steadfast.
self-confident (adjective)
sure.
tenacious (adjective)
brave, enduring, fervent, hard, obdurate, obstinate, stiff, stony, strong, undaunted, unflagging, bulldogged, flintlike, bold, bullheaded, courageous, dauntless, determined, devoted, diligent, dogged, faithful, headstrong, hearty, indefatigable, indomitable, insistent, persevering, persistent, plucky, resolute, spunky, staunch, steadfast, strong-willed, stubborn, tenacious, tireless, tough, true.

n.

assertive, dignified, proud, self-assured, self-confident, self-possessed. causal, causative, driving, emphatic, enabling, forceful, formative, instrumental, productive. comfortable, competitive, decide, devastating, friendly, handsome, hardball, hot, hotly. authoritative, definitive. crucial (noun)
definitive.
decisive (noun)
conclusive, critical, crucial, deciding, definite, determinant, determinative, determining, fatal, fateful, important, peremptory, resolute, unhesitating.

Other synonyms:

driving, emphatic, formative. authoritative, causal, causative, enabling, hardball. hotly, instrumental, productive. competitive, definitive, devastating, friendly. handsome. comfortable. hot. crucial
crucial.
Other relevant words:
assertive, authoritative, causal, causative, competitive, critical, crucial, decide, deciding, definitive, determinant, determinative, determining, enabling, fatal, fateful, forceful, formative, friendly, hardball, hot, important, peremptory, self-assured, self-confident, unhesitating.

Usage examples for decisive

  1. During this year no decisive battle was fought. – Sages and Heroes of the American Revolution by L. Carroll Judson
  2. The decisive moment had arrived. – The Captain of the Gray-Horse Troop by Hamlin Garland
  3. Mercy advanced to the surgeon, and put the decisive question to him. – The New Magdalen by Wilkie Collins