Non-return valve for windscreen washer hose with an internal diameter of 4mm or 5mm.
(It is a tight fit into 4mm hose, so will be difficult to insert if your hose has hardened with age: you could try leaving the end of the hose in hot water for a few minutes to soften it before fitting).
When fitting, please note that there is an arrow in the middle of widest part, to indicate direction of the water flow.