noun

the quality of being determined; firmness of purpose.

those who succeed because of sheer grit and determination

resolution,resolve,resoluteness,will power,strength of will,strength of character,single-mindedness,sense of purpose,firmness of purpose,fixity of purpose,purposefulness,intentness,decision,decidedness,steadfastness,staunchness,perseverance,persistence,indefatigability,tenacity,tenaciousness,staying power,strong-mindedness,backbone,the bulldog spirit,pertinacity,pertinaciousness,stubbornness,doggedness,obstinacy,obdurateness,obduracy,inflexibility,spiritedness,braveness,bravery,boldness,courage,courageousness,pluck,pluckiness,stout-heartedness,Dunkirk spirit,sitzfleisch,guts,spunk,grit,stickability,stick-to-it-iveness,intension,perseveration