Is it flammable or inflammable?
It is true that ‘flammable’ and ‘inflammable’ have the same meaning of ‘highly combustible’.
In English grammar, we frequently see the prefix ‘in-’ to negate something (‘visible’ becomes ‘invisible’; ‘capacity’ becomes ‘incapacity’).
Because of that widespread use of the prefix in- to negate words, ‘inflammable’ is open to misinterpretation as if it were a negative word.
However, in ‘inflammable’, the ‘in-’ element is an intensive and not an expression of negation.
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/metaphorical usage, only ‘inflammable’ is used (an inflammable temper).