\[ \cos \] makes no sense to me! If \[n\] is taking on integer values, what values do you get for \[\cos\]? Try it on your calculator!
On the other hand if you take a time-domain signal like \[\cos(t)\] and sample it every \[T_s\] seconds then you can write \[\cos(n T_s)\]. Note that the period of \[\cos(t)\] is \[2 \pi\] so you have to choose \[T_s\] to be much smaller that this to have proper discritezation.