Passing off is a cause of legal action in English intellectual property law which is sometimes referred to also as "unregistered trade mark" law and by some as a roundabout way of implementing unfair competition law in the UK (which there isn't, expressly; though now the EU has standardised anti-trust regulations Community-wide.)
The action allows a claim by one person or business against another legal or moral individual who has used a mark or symbol associated with the plaintiff in a way not necessarily permitted by him. Following Lord Oliver's judgement in the case of Reckitt & Colman Ltd v. Borden Inc, there are three things that a claimant must prove in passing off for a successful action:
1. That the claimant had "goodwill" in the mark in question. What "goodwill" is, in this sense, is, according to Inland Revenue v. Muller, the "thing which attracts customers to a business." This can be just about anything whatsoever, although in some things it's harder to prove that one has goodwill (shapes and colours in particular, although it has been done.) Goodwill can be had in a description of a product, assuming the product is particularly unusual; in the case of Reddaway v. Banaham, the product in question was "Camel hair belting." This is also illustrative of the concept of secondary, or acquired goodwill, which is a secondary meaning which can attach to words through use and market penetration; for instance, the word "camel" may be referring to a foul-tempered, behumped animal that spits a lot or a brand of cigarettes. A mark does not need to be a registered trade mark to be protected by passing of, nor does it need not to be a registered trade mark. Indeed, it's not uncommon in cases of trade mark infringement for the plaintiff to also claim in passing off as well, as upon registration all trade mark applications are required to be distinctive and simply by virtue of having a registered trade mark one can argue that one has goodwill in that mark.
2. That the defendant made a misrepresentation using the mark in which one has goodwill. Misrepresentation can be a wide variety of actions; basically it's engaging in any activity under a logo or brand that's moderately similar to that of the plaintiff. The misrepresentation MUST relate to the goodwill owned by the plaintiff, it is not permitted for a plaintiff to attempt to police others' goodwill in passing off. There is also no requirement for a common field of activity; in the case of Harrods v. Harrodian, the claimant, an upscale department store, sued the Harrodian school claiming encroachment onto their goodwill in the name "Harrods." The claim was that it was misrepresenting their school as being somehow linked to Harrods when there was no such link. Another such case was that of Stringfellows v. McCain in which the claimant was a strip club and the defendant manufactured oven chips. As we shall see, in cases where there is no common field of activity very often an argument used is that the defendant is undergoing unjust enrichment by benefiting from the goodwill in the claimant's name (though why the Cabaret of Angels would endorse a potato product is rather unclear) and as a result this leads some to claim that passing off is a form of unfair competition law. (As an aside, the court upheld the unjust enrichment claim in Stringfellows.)
3. The plaintiff must be able to show some sort of damage. Damage in passing of can come in all types. Strictly speaking, the main form of damage that a plaintiff might suffer is lost sales due to consumers, for instance, believing that they are getting one manufacturer's product when they are getting another. In such instances of lost sales, the rule is to take one's consumer as one finds them - in the case of Edge v. Nicholls, the consumers were illiterate washer women, so the fact that the products (brands of washing powder) were labelled as being different was held to be immaterial, as the packaging looked remarkably similar. For assessing damage in the form of lost sales or confusion of end users, as in registered trade mark law, an objective model is taken of the average user of such products. This could be the proverbial "man on the Clapham Omnibus" in the case of everyday or commonplace items, but for scarcities or specialist goods, the average consumer of those goods is used for this test. Another form of damage is the loss of opportunity, whereby due to the actions of the defendant, the plaintiff lost a potential chance to expand his business. Often this is used where there is no common field of activity, as per Stringfellows v. McCain above, though as I have already stated, how a strip club would expand into potato products is beyond my ken. It is still unclear as to whether the damage has to tie in to the misrepresentation; though the consensus at the moment is that really it should.
Of course, the Reckitts approach is not the only one available; a similar House of Lords case decided at the same time, Erven Warnink BV v. J. Townsend & Sons, also known as the "Advocaat" case because of the nature of the product involved in the litigation, sets out a different approach in the judgement of Lord Diplock. This approach is arguably narrower, and as well as the damage, misrepresentation and goodwill requirements of the standard test, also adds necessities such as the passing off being committed "in the course of a business" and that it must be done towards the potential customers or consumers of the plaintiff's rather than just in the marketplace in general.
Passing off can also occur in reverse, where a defendant misrepresents the plaintiff's goods or services as being his own, though this is very rare. In the US, a high-profile instance of this is the case of Dastar Corporation v. 20th Century Fox.
Sources: Cases and statutes as cited.
Yes, that's how you spell "trade mark" in England. "Trademark" is the American spelling.