I was sure I could settle the question of exactly what jam and jelly is and Lo! I found the definitive source.

Explanatory Notes to the Harmonized Commodity Description and Coding System is a very scholarly and universally accepted sourcebook on the meaning of terms used in the Harmonized Tariff System, which is the classification of goods for import or export used by nearly every country in the world to categorize goods so they can be declared and duty assessed. Chapter 20 covers prepared foodstuffs and heading 2007 covers jams, jellies and other preparations of fruits. Only jams and jellies are mentioned; the word "preserves" is not.

The Explanatory Notes saith:
Jams are made by boiling whole fruit or fruit pulp or certain vegetables (e.g., marrows, aubergines) or other products (e.g., ginger, rose petals) with sugar in approximately equal proportions. When cool they are of moderately firm consistency and contain pieces of the fruit.
Marmalades are a variety of jam generally prepared from citrus fruit.
Fruit jellies are prepared by boiling fruit juices (expressed from raw or cooked fruit) with sugar until the product sets on cooling. They are of firm consistency and free from pieces of fruit.


In the U.S., the word "jam" has the meaning noted above, but it is not used as nearly as frequently as in UK English, and it is rare that it is called that on the label of a jar (except, of course, imported jams). A domestic jar of jam is invariably labelled as "strawberry preserves", or "apricot preserves" or whatever.

BTW, I expect I shall use the Explanatory Notes again some time to settle a question of exactly what something means. It's a really fascinating work and has legal authority. I spent hours once learning about fabrics, and the exact description (often with pictures or drawings) and meaning of words like twill, denim, gabardine, jersey, selvage, orlon, ramie etc.