A break in the rock record where layers representing different periods in time intersect - leaving gaps in time between to the two intersecting layers. For example, due to geologic upheaval, a 400 million year old layer of granite may have been desposited on top of a 600 million year old layer of sandstone. The respective layers represent a gap of 200 million years in geologic terms, yet they are intersecting. This phenomenon is called unconformity.
The Superstition Wilderness in Phoenix, Arizona is filled with geological unconformity.