It's described to be the smallest diameter of the portion of the tapered leader. To avoid confusion, the leader is the material located between the fly and fly line. It's a material made of monofilament and is usually tapered so that the fly doesn't mess with the fly line when it is thrown.
Now that were clear on what tippet and leader is, let's go to how to tie a fly to the tippet. This may seem like a simple task to do before fishing but there are times when tying could take forever. Expect for days like these to happen and just in case it does, be sure you have few fly-tying-to-tippet knowledge up your sleeves.
You can also practice tying a fly to your tippet at home before you conduct a fishing activity outdoors. You can use an ordinary rope or a clothesline you found in your house. This step is important because when you start tying both ends of your leader and tippet, you need to make sure that the diameter or space movement is just right for your fly. When you first practice, you can start using a string with a larger diameter just so you get the hang of tying both open ends.
To avoid deformity, you can use oil or similar liquid to moisten the knot. The final step in tying a fly to the tippet is done by trimming or cutting the loose ends you used for tying the knot but leave an allowance. When you do this, make sure you leave an allowance of about 1/8 inch so the knots that you made won't easily disjoin.
After you have done the tying part on a practice rope, you can try tying real materials which are the tippet and leader. Make sure that you don't tie up the two too tightly because it's important for it to have a space for movement. And lastly, whenever you're ready, you can now test your knot right before fishing to make sure that the knot won't slip. If it does, you can easily cut the knot off and make another one.