C¶

The default generated parameter of DSolve, RSolve, Reduce, and Solve. Solutions with free constants are expressed in terms of C[1], C[2], …

$ wo "DSolve[y'[x] == y[x], y[x], x]"
{{y[x] -> E^x*C[1]}}

Since C is a built-in symbol it is Protected, so it cannot be used as a variable.

$ wo 'Attributes[C]'
{NHoldAll, Protected, ReadProtected}

$ wo 'C = 12'

Set::wrsym: Symbol C is Protected.
12

Use Unprotect to override the protection:

$ wo 'Unprotect[C]; C = 12; C'
12