A short period of cold temperatures. It was finally beginning to get warm outside when we were hit with a cold snap that damaged my newly blossoming tulips.
Also, cold spell. A short period of unusually cold weather, as in The recent cold snap has threatened the crop. The first expression presumably likens snap in the sense of "a sudden bite or cut" to sudden unexpected cold. The variant is more obvious, spell having been used in the sense of "a bout or turn at something" since the early 1700s. [Early 1800s]