Boot, Tyre Boot

A firm but flexible piece of anything that can be inserted between a tyre and a tube, to provide additional support at a damaged point. If the tyre is cut, gashed, or torn to the point your tube is visible, it will puncture again in short order, and that puncture is likely to be explosive not gradual.

from Rear Tyre torn at rim - what was the cause?

Also, small holes in the tread can give punctures too, as the tube bulges out and wears off on the road surface.

Park makes a boot, called the TB-2 which is similar to an oversized sticker pre-glued tube patch, but is intended to stick to the inner wall of the tyre.

However the enterprising cyclist can use many different items as a boot.

• An empty gel wrapper is a common choice (but clean it well, the content will go everywhere even if you think it is empty!)
• If your country uses polymer banknotes they can work, but not paper notes,
• Any item of flexible plastic from your tools
• Strips of duct tape

Tubeless tyres might benefit from a boot, if the damage is too large for the sealant to plug. However it could be hard to get the boot to lie in the right place for long enough.

Relevant questions: What's the point of tyre boots?