Reverse

The rule on opening bids is - length over strength. Therefore, the junior suit might be bid first.

If your partner does not support your opening suit, you will bid your second suit, showing 5 cards in the first suit and 4 in the second. This may involve bidding at one level higher than if the senior suit was bid first.

When the senior suit is bid second at the 2 level, it is called a reverse and requires an opening hand of at least 15 points. 

If the opener does not have a hand strong enough to reverse she must rebid her first suit.

It is important to note that a senior suit bid second at one level is not a reverse.