A lumbar puncture (spinal tap) is an invasive medical procedure where a needle is inserted into the spinal canal (subarachnoid space) to remove cerebrospinal fluid (CSF) for analysis, to inject medication into the subarachnoid space to alleviate pain, or to inject other medication to treat diseases of the meninges, such as to deliver chemotherapy for cancer that has metastasized to the meninges.

Cerebrospinal fluid is a liquid in the subarachnoid space that stabilizes the brain and spinal cord and keeps them from moving. Any abnormalities of the meninges, spinal cord, and/or brain can affect the cerebrospinal fluid. That is why removing cerebrospinal fluid for examination is important if abnormalities of the meninges and/or spinal cord is suspected.