Periodic
Remember that constant functions like Y=2 and the like are periodic for CT and DT.
For all Y(t) and Y(x) Y is equal to 2. The period can be any integer greater than except 0.
Aperiodic
An asymptotic CT function like $ f(t)={ 2 \over(t-3)} $ is aperiodic.
There exists no number N where $ f(t) = f(t+N) $.
