Hyperbolic discounting is a cognitive bias in which an individual shows a preference for a reward that arrives sooner over a reward that arrives later. For example, offered the choice between $50 now and $100 a year from now, many people will choose the immediate $50; however, given the choice between $50 five years from now and $100 six years from now, most people choose the $100 in six years, even though the second set of choices is similar to the first set in that there is one year of time in between the immediate and the delayed reward (1)(2).
In hyperbolic discounting, reward valuation falls rapidly with small periods of delay and then slowly for longer delay periods. This is in contrast to exponential discounting, in which reward valuation falls by a constant value per unit time of delay, regardless of the length of delay.

Sources:

(1) Green, L.; Fry, A. F.; Myerson, J. (1994). “Discounting of delayed rewards: A life span comparison”. Psychological Science 5 (1): 33–36

(2) Kirby, K. N. (1997). “Bidding on the future: Evidence against normative discounting of delayed rewards“. Journal of Experimental Psychology: General 126 (1): 54–70.