Simply, it is when an object is offered for sale as being made by the original manufacturer, when in fact it wasn't. Stamps, coins, mechanical banks, toys and hundreds of other items have been victims of forgeries.
Cannons are no exception.
Sometimes a forgery doesn't start out that way. There is nothing wrong with making antique reproductions. The seller simply needs to state that. However, when a seller knowingly sells an item as being an original, when it is not, this is when the item is called a forgery. If the seller states that it is not an original, then it is a reproduction.