A large heavy oblong shield protecting the whole body.
Archers and crossbowmen would crouch behind a pavise while reloading. A row of such shields is called a pavisade.
Etymology: Named after Pavia, Italy, where the shields supposedly originated (somewhere around the middle of the thirteenth century).
This link has many fine examples of historical pavises. Some have a distinct central ridge: it housed a stake that was used to prop up the shield. Some fighters had a pavisier to hold the shield for them during battle; others used the stake to hold the shield in place.
For a modern example of a pavise, below is a picture of one my friend built for his wheelchair (not in this picture is the energetic and colorful battle going on around him).