Real Dictionary

second wind
singular noun: When you get your second wind, you become able to continue doing something difficult or energetic after you have been tired or out of breath.

second wind in American English
the return of relatively normal ease in breathing following the initial exhaustion that occurs during severe exertion or exercise, as while running
noun: the return of ease in breathing after exhaustion caused by continued physical exertion, as in running

second wind in British English
noun: the return of the ability to breathe at a comfortable rate, esp following a period of exertion