Spatial Frequencies
Usual frequency = cycles / time (units: s-1 or Hz)
Spatial frequency = cycles / distance (units: m-1). It's a wave in real space.
Spatial frequency is introduced and depicted in the first k-space web tutorial.
Further reading on this topic:
Books: MRI The Basics p165, Q&A in MRI p95f
Online: e-MRI 1, 2, 3