The Merriam-Webster Dictionary quite simply defines fuelwood as “wood grown or used for fuel.” This compound word is thought to have come into use in the 14th Century.
Whereas with kindling, you are looking for twigs no larger than a pencil, with fuel wood, there isn’t an actual limit to how large a piece might be, but there are certainly practical limits. A general rule of thumb that we use in our outings is that a good size for fuel wood is about the thickness of your wrist, or, perhaps, your forearm if you’re building a larger fire. If you have pieces larger than this, they should probably be split down to a smaller size before being used to feed the fire. You’ll want your initial pieces of fuel wood to be smaller.
Where the tinder is used to alight the kindling, the kindling is used to start fuel wood on fire. Once a campfire is established, additional pieces of fuelwood can be used to sustain it.