'c' is a mathematical binary infix operator. (n c r) is the number of sets with r elements which can be made with n atoms. (It can also be written C(n, r), or nCr, or even nCr.)
Both n and r must be nonnegative integers, and r must be less than or equal to n. For example, (4 c 2) is the number of sets with 2 elements which can be made from 4 atoms. Say the 4 atoms are w, x, y, and z. The possible sets are: {w, x}, {w, y}, {w, z}, {x, y}, {x, z}, and {y, z}; (4 c 2) is 6. (n c r) equals (n! / ((n - r)! * r!)), which equals ((n p r) / r!).
(n c r) equals the (r + 1)th number on the (n + 1)th line of Pascal's triangle, which has many interesting properties. For example, in Eindhoven notation, (+ : (i ∈ Z) ∧ (0 ≤ i ≤ n) : n c i) = 2n.
Phssthpok says the 'c' stands for 'choose'… if you have n items available and choose r of them, the number of choices available is (n c r) and it reads as 'n choose r'.
N-wing tells me maybe the infix 'c' should be capitalized, (n C r). He also says it stands for 'combination', like p (or P) stands for 'permutation'.