A rain liner for a jacket is a detachable layer that can be added to the inside of a jacket to make it waterproof or water-resistant. Typically made from materials like Gore-Tex or other synthetic fabrics that are designed to repel water, rain liners are lightweight and breathable to ensure maximum comfort and functionality.

The liner is usually attached to the inside of the jacket with zippers or snaps, and can be easily removed when the weather is dry. Rain liners are commonly found in outdoor and sportswear jackets, and are especially useful for activities like hiking, camping, and biking where wet weather is a possibility.

When choosing a rain liner for your jacket, it is important to consider factors such as the type of fabric, the level of waterproofing or water resistance, and the fit of the liner to ensure that it provides adequate protection from the elements while still allowing for ease of movement and breathability.