A lumbar puncture (an LP) is the insertion of a needle into the fluid within the spinal canal. It is termed a "lumbar puncture" because the needle goes into the lumbar portion (the "small") of the back. Other names for a lumbar puncture (an LP) include spinal tap, spinal puncture, thecal puncture, and rachiocentesis.
An LP is most commonly performed to diagnose a disease, namely to obtain a sample of the fluid in the spinal canal (the cerebrospinal fluid) for examination.
An LP can also be done to treat diseases, as a way of administering antibiotics, cancer drugs, or anesthetic agents into the spinal canal. Spinal fluid is sometimes removed by LP for the purpose of decreasing spinal fluid pressure in patients with uncommon conditions (such as, for examples, normal-pressure hydrocephalus and benign intracranial hypertension).