Although seems similar, Competitive Bidding vs Defensive Bidding is a different creature. Actually they are the opposite. In a casual conversation, a bridge player probably uses both terms interchangeably to refer to the effort to counter the opponent. In a way, it is actually not entirely wrong, although it is not precisely right either.
When the opponent has started the bidding and we do overcall or takeout double, we are doing a competitive bid. In other words, we try to be competitive. We try to get in into the auction. Hence the term.
Therefore, we can think competitive bidding as an effort we made to get the bidding that has started by the opponent in order to win the contract our way.
Some other examples: Michael’s Cuebid Unusual 2NT, Leaping Michael, Responsive Double, Capelletti, DONT, etc.
On the other side of the coin, if your partner did the opening bid and the RHO (Right-Hand Opponent) overcalled, 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 own bidding system. In other words, defensive bidding is all the bid we do to counter opponent’s competitive bidding.
Some other examples: 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. But from now on, I hope you do not get confused with these two terms explained here.