What is the terrible triad of O'Donoghue?

The terrible triad of O'Donoghue is concomitant injury to the medial collateral ligament, anterior cruciate ligament, and medial meniscus, caused when a valgus load is applied to the externally rotated knee with the foot planted. This is the classic football clipping injury.
Lateral meniscal tears, as opposed to medial meniscal tears, are actually more commonly associated with anterior cruciate ligament tears.