A recliner is a chair in which you can lean back at different angles. Many also lift a rest for your legs. People buy them for watching television, reading, or napping; it is an everyday furniture word, a little informal. The related verb is recline: when you recline, you lean back, and a recliner is the chair that lets you do that. Shops sometimes say reclining chair, which is the same idea. It is not a rocking chair, which moves forward and back, and it is not simply a sofa, though some sofas have reclining seats.