Miriam-Webster says it's "the quality that allows someone to continue trying to do something even though it is difficult or unpleasant".
Wiktionary puts it even more simply, "(informal) persistence; determination".
Do not plan for ventures before finishing what's at hand. ~ EuripidesWhat are your secrets for sticking to something you've started? When do you decide a project isn't worth the effort? Do you think it's important to finish no matter whay?
It's the job that's never started as takes longest to finish. ~ J. R. R. Tolkien
Whenever you have taken up work in hand, you must see it to the finish. That is the ultimate secret of success. Never, never, never give up! ~ Dada Vaswani