What is tortious interference with contract?

First, let's get the name right.  It is not "tortuous" interference with contract.  It is "tortious" interference with contract.  (These cases can indeed be "tortuous" to handle.  But that is not the name of the cause of action.)

A tortious interference cause of action arises when (a) there is a valid contract or business expectancy between the plaintiff and a party other than the defendant, (b) someone who is not a party to the contract (the defendant) has knowledge of its existence, (c) the defendant intentionally causes the contract to be broken (breached) by one of the parties to the contract (not the plaintiff) and (d) the breach of contract causes damages to the plaintiff.  If each of these elements are present, a tortious interference with contract case may exist.  If the contract in question was "terminable at will," meaning that either party had the legal right to break it, a case may still exist.  However, an additional showing must be made by the plaintiff that the defendant used "improper methods" to interferfere with the contract that was terminable at will.  This same additional requirement applies to legal relationships that are in the category of business "expectancies" but that do not rise to the level of a contract.  Business expectancies may also be interfered with in a way that leads to liability for damages.  But improper methods of interference must be shown.