Combustible, inflammable, flammable, incendiary, inflammatory, which are not close synonyms, since they apply to different types of nouns, are comparable because they mean showing a tendency to catch or be set on fire.
Combustible applies chiefly to material which is of such a nature that it catches fire easily and usually burns quickly.
Inflammable applies to materials ready to flame up at the slightest cause; the term, however, suggests more than the ease with which a thing burns, for it stresses a capacity for flaring up dangerously.
Inflammable as applied to persons, their natures, their hearts, and their temperaments stresses the danger as well as the ease in arousing them (as to anger, excitement, or passion); it applies also to a subject or situation that can induce such arousal.
Flammable is equivalent to inflammable and used chiefly in respect to materials dangerously likely to flare up or explode.
Incendiary not only stresses a capacity for starting up or causing a conflagration but a malicious intent to do the same. Therefore the term applies not only to something that is designed to cause fire but to fire that is caused by intent or design.
The term may be extended to something (as an act, practice, speech, or publication) that tends to arouse something (as sedition, evil passions, or violence) that acts as destructively or disastrously as fire.
While the adjective is seldom applied to persons, the noun incendiary chiefly, except when used as short for such phrases as incendiary bomb, designates a person who maliciously or willfully sets fire to a building or other property or who deliberately incites quarrels, unrests, sedition, or violence.
Inflammatory, like incendiary, emphasizes the power to cause a fire, especially in the hearts or minds of people, but it carries less suggestion of a malicious intent.