Is it flammable or inflammable?

It is true that ‘flammable’ and ‘inflammable’ have the same meaning of ‘highly combustible’.

In English grammar, we frequently use the prefix ‘in-’ to negate something (‘visible’ becomes ‘invisible’, ‘capacity’ becomes ‘incapacity’). However, it is an intensive here and not an expression of negation.

Because of that widespread use of the prefix in- to negate words, inflammable is open to misinterpretation as if it were a negative word.

Inflammable really means able to be inflamed (inflame+able). Something which cannot be burned is nonflammable.

Since this word is often on warnings which must have absolute clarity at their first reading, the preferred terms for warnings and all technical writing are: ‘flammable/highly flammable’.

In figurative usage, only ‘inflammable’ is used (an inflammable temper).