Is there a way to tell CAS that a variable is a constant?

[parisse]

I don't understand what constant would mean. For example if a is a free variable then integrating some expressions containing a with respect to x will consider that a is a constant (w.r.t. x). But as explained in another thread, definite integration is harder for expressions with parameters, and you might need assumptions otherwise the system does not know the limit at the endpoints (for example int(exp(-a*x),x,0,inf) does not converge if a<=0).