The amount you pay towards a claim before the insurer pays anything.
On private medical insurance the excess is usually annual rather than per claim, so you pay it once in a policy year however many eligible claims follow. Some policies apply it per condition instead, which can mean paying more than once in a year. Raising the excess lowers the premium because you are taking on more of the cost yourself. Check the policy wording for whether the excess applies to outpatient treatment, inpatient treatment, or both.