A run is a combination of playing cards where cards have consecutive rank values. Some games, such as cribbage

Cribbage

Cribbage, or crib, is a card game traditionally for two players, but commonly played with three, four or more, that involves playing and grouping cards in combinations which gain points...

, will specify that an Ace counts as one ("ace low"); others will specify that an Ace counts above a King ("ace high"); yet others, such as poker

Poker

Poker is a family of card games that share betting rules and usually hand rankings. Poker games differ in how the cards are dealt, how hands may be formed, whether the high or low hand wins the pot in a showdown , limits on bet sizes, and how many rounds of betting are allowed.In most modern poker...

, allow an Ace to count high or low, but not both in the same run.

Examples

Ace high
Ace low