Breach of contract means failing to perform any term of a contract without a legitimate legal excuse.
Breaking a contract entails followed the legally defined procedure (as per the terms of the contract’s break clause) to terminate the contract.
To elucidate: a breach on the part of one party would likely be grounds for breaking the contract.
Efficient breach IS considered a “break” of sorts, since the intent is the same. If one cannot comply with contract’s break terms, and considers it more profitable to suffer the fallout of a breach than to fulfill the contract, his would be an “efficient breach.” In a way, you could view it as a break of contract, with an added penalty for not complying with the break terms. Efficient breach and stam breach are NOT synonymous, though. Conceptually, the idea of efficient breach is closer to the definition of contract break than it is to that of contract breach.