Although similar, Competitive Bidding vs Defensive Bidding is different creature. Actually they are the opposite.
When the opponent has started the bidding and we do overcall or takeout double, we are doing a competitive bid. So, we try to be competitive. We want in. Hence the term competitive bidding.
Therefore, we can think competitive bidding as an effort we made to get the bidding that has started by opponent in our way.
Some other examples: Unusual 2NT, Leaping Michael, Responsive Double, Capelletti, DONT, etc.
On the other side of the coin, if the partner did an opening bid and the RHO overcall, we need to do something to let partner know about our respond of the opening. You may do a negative double, or show your suit with “Negative Free bid, etc”. These are examples of “Defensive Bidding”.
So, defensive bidding is an effort in our part to overcome opponent’s interference. Back to the realm of our system .
Some other examples of defensive bids: D0PI/R0PI or DOPE/ROPE convention, Lebensohl, Redouble, fit-showing cue-bid, Support Double, etc to name a few.
So, when one partnership do a competitive bid, their opponent will do the defensive bid. In a way, they work as opposite interactively. That’s it.. that’s the difference between Competitive Bidding vs Defensive Bidding