A crotalus, also known as a crotalum or clapper, is a wooden liturgical rattle or clapper that replaces altar bells during the celebration of the Tridentine Paschal Triduum at the end of Lent in the Catholic Church. It is also occasionally used during the celebration of the Ordinary Form Roman Missal during the Paschal Triduum, but its popularity decreased following the liturgical reforms of the Second Vatican Council. The crotalus is also sometimes used in Episcopal Church parishes.

A clapper is a basic form of percussion instrument. It consists of two long solid pieces that are struck together producing sound. They exist in many forms in many different cultures around the world. Clappers can take a number of forms and be made of a wide variety of material. Wood is most common, but metal and ivory have also been used. The plastic thundersticks that have recently come to be popular at sporting events can be considered a form of inflated plastic clapper.