Non-flammable materials are by definition non-combustible or not easy to set on fire, according to Dictionary.com. On the NAFPA fire rating system, these materials have a rating of zero and do not burn, even when heated. Examples of non-flammable liquids include water and carbon tetrachloride.
The words non-flammable and inflammable are often confused. Inflammable is a synonym for flammable. In this case the prefix "in-" is not a negative prefix but an intensifying one. While the use of the word inflammable is more common in Great Britain than in the United States, it causes confusion in both locations.
Materials with a NAFPA rating of 1 burn if preheated. These materials have a flash point above 200 degrees Fahrenheit. Canola oil has this flammability rating.
NAFPA 2 rated materials require heating or a high ambient temperature before they ignite. Diesel fuel and other level 2 materials have a flash point between 100 and 200 degrees Fahrenheit.
A solid, liquid or gas in flammability category 3 has the potential to ignite under almost all normal temperature conditions. Flammable liquids in this category have a flash point below 73 degrees and a boiling point over 100.
Flammable gases or volatile flammable liquids have the potential for spontaneous combustion at normal ambient temperatures. These materials are a category 4 using the NAFPA rating system.