That if something is categorically imperative, that you have a duty to act from it regardless of convenience?
Actually, that's not entirely clear... but it's just a question of how we define "deontological" rather than any deep philosophical thing. Contemporary ethical discussion seem to love the term as a contrast with consequentialist theories or virtue theories. Moreover, Kant could not have used the term because it did not exist until after he was dead.
Yes, the CI is such that it obligates the moral agent to perform its duty and since it relates duty to morality, it is "deontological".
But then if we look carefully, doing your duty is not enough for Kant, the maxim of the action of your duty must be universalizable and in accordance with pure reason. There's a lot of hoops there, but to say it another way, if my duty were somehow to help an old lady across the street, I only do my duty for Kant when I help the old lady across the street from the motive of duty itself (rather than allowing some emotion or desire for praise to be the source of my duty).
This term is less helpful than the counterpart consequentialism, because different consequentialisms differ not primarily in the methods -- calculating the best consequence given some limitations but in the quantity that matters. Deontologies differ in the way that duty relates to morality at all (is it performance? is it rights-based? Are these duties we contract either through some social contract or as a condition of rational action in a world with multiple moral agents?) In many contexts, the differences in forms matter greatly.