Are "of the order of n" and "Big O" the same thing? I am learning from the MIT course Introduction to Algorithms.

The professor says:


  Now, remember $\Theta(n)$ is essentially something
      that says "of the order of $n$".


What does "of the order of $n$" mean? Are "of the order of $n$" and "Big O" the same thing?

Please provide the name of any widely recognized textbook that contains detailed explanation about this.
"On the order of" is an informal statement which really only means "approximately". Big O notation is a precise mathematical formulation which expresses asymptotic behavior, not approximate values of a function (e.g., $10n \in O(n)$, despite $10n$ being 10 times as larger as $n$). They can hardly be considered the same things. What the lecturer is trying to do here is supposedly give you some (hand-wavy) intuition as to how Big O notation works, but that should not replace you actually sitting down and learning the precise definitions.

As for your request regarding further reading on Big O notation, (in addition to the linked Wikipedia article) I suggest you take a look at our reference question.



An addendum: After listening to the video again, you can make out the lecturer say "of the order of $n$". This is the British English version of American English "on the order of" and carries the same meaning.