A typesetting symbol which has several distinct meanings in mathematics. It is used for a number of purposes.
1. To denote Boundaries between elements in a list, as in 
.
2. To delimit indices in the element of a matrix, as in 
(although it is frequently omitted
when implied by context).
3. To indicate the comma derivative of a tensor.
4. In place of a decimal point in continental Europe, e.g., 3,14159.
