With acknowledgement to the Victoria & Albert Museum, London, the article reproduces the following brief definitions:
What is a fake?These definitions should come with a caveat that, while they are handy when discussing protection and enforcement issues with clients, the words are not cast-iron terms of art that are understood in all jurisdictions as having the same meaning. In particular, the notion that a work in the style of a particular artist is a "copy" is a controversial and dangerous one. The courts in England and Wales have held, in Gordon Fraser Gallery Limited v Tatt  RPC 505, that there is no copyright in a style. While the adoption of a specific style, in combination with other factors, may constitute a misrepresentation or a fraud, it is not a copyright infringement. If the contrary had been the case, the world would have enjoyed the paintings of rather fewer impressionists and cubists.
… an object that has been tampered with – e.g. a signature has been added or false indications of the object’s history have been introduced with the intention to defraud and increase the item’s value.
What is a copy?
… a direct replica of a pre-existing work or a work that imitates or was created in the style of a given artist. If the work is in the public domain, it is not illegal to make a copy of it provided there is no attempt to deceive or make anyone believe it is an original work. To reproduce a contemporary work protected under copyright law requires the permission of the artist whose work is being copied.
What is misattribution?
… this arises when a mistake is made in determining the original artist of a work and typically occurs when works have been restored and original details masked. This is considered a genuine mistake as there is no intention to deceive.
What is a forgery?
… an object that is created from scratch with the intention to deceive – it is a fraudulent imitation of an existing work.
What is fraud?
… “the act of making people believe something is not what it really is for criminal benefit".
You can read "Modern masters under threat" in full here.