This Latin phrase means 'agreement to the same things', or a 'meeting of the minds'. In contract law, consensus ad idem is required to the extent that the parties have reached agreement on at least those terms without which there cannot be a workable transaction, and on any other terms that either party has indicated must be agreed on before the contract is made.
The phrase 'consensus ad idem' is commonly abbreviated to 'consensus' but the meaning remains the same.