一针 is a noun meaning 'a single injection' or 'one shot (of a vaccine/medicine)'. It's used in medical contexts to refer to one dose administered via syringe. The word is a compound of 一 ('one') and 针 ('needle'), but as a fixed term it specifically refers to the injection itself, not the needle. You'll hear it in phrases like 打一针 ('get a shot') and 一针疫苗 ('one vaccine dose').