【Fortitude】 , 【grit】 ,  【backbone】 ,  【pluck】 ,  【guts】 ,  【sand】  denote a quality of character combining courage and staying power.

【Fortitude】 stresses strength of mind and firmness of purpose; it implies endurance, often prolonged endurance, of physical or mental hardships or suffering without giving way under the strain.

【Grit】 also implies strength and firmness of mind, but it stresses an incapacity for being downed by difficulties or hardships and usually also suggests both a willingness to suffer the privation and pain necessary to the attainment of one's ends and the 【fortitude】 to bear them.

【Backbone】 emphasizes resoluteness of character; it implies either the ability to stand up in the face of opposition for one's principles or one's chosen objectives, or determination and independence that require no support from without.

【Pluck】  implies a willingness to fight or continue fighting against odds; thus, it is  【pluck】  that keeps a sick person at work; it is  【pluck】 that keeps soldiers from retreating in the face of disaster.

【Guts】 , which is often considered expressive but not entirely polite and is therefore sometimes facetiously replaced by intestinal 【fortitude】 , stresses possession of the physical and mental vigor essential both to facing something which repels or frightens one and to putting up with the hardships it imposes.

【Sand】  comes close to  【grit】  in its meaning, but since it often carries a suggestion of 【pluck】 or of the ability to fight against odds, it does not so strongly as  【grit】 connote triumph over difficulties.