Lumbar cistern:

cerebrospinal fluid filled area between the end of the spinal cord and the end of the vertebral column. Cerebrospinal fluid is often sampled here in a process (i.e. lumbar puncture or spinal tap) that can be used to diagnose a variety of conditions such as meningitis and multiple sclerosis.

Watch this 2-Minute Neuroscience video to learn more about the spinal cord.