All coolants are flammable, but not all are combustible. The difference between the two is that coolants that are only flammable will burn when they come into contact with a heat source, while combustible coolants will actually catch fire and continue to burn even after the heat source is removed. This makes combustible coolants much more dangerous to use than their flammable counterparts.
Most commonly used coolants are based on water or glycol, and these are generally considered to be non-combustible. However, there are some exceptions - for example, ethylene glycol can be quite combustible if it becomes concentrated enough through evaporation. In general though, you can consider most water or glycol based coolants to be safe from the risk of combustion.
There are some synthetic coolants available which are specifically designed to be highly flame retardant, and these may be a better choice for applications where there is a risk of exposure to high temperatures or naked flames. If you're unsure about which type of coolant to use in your situation, it's always best to consult with a professional who can advise you on the best product for your needs.