I am trying to find the word that means that follows: A question or instruction is given, but no reply is made. The assumption is that since no one has replied that it is okay to go ahead. For example "Can I have next Tuesday off as annual leave?" Since the manager did not come back and say "No", my assumption was that it was okay. I know that there is a word for this, but cannot remember what it is.
This is not a single word, but you are applying implied consent:
Implied consent is consent which is not expressly granted by a person, but rather inferred from a person's actions and the facts and circumstances of a particular situation (or in some cases, by a person's silence or inaction).