Optimal is an adjective only: "the optimal route". Optimum is both a noun and an adjective — you can reach an optimum, or take the optimum route. Where both fit, either is correct. What neither means is "very good": an optimum is the single best point under the given conditions, so "more optimal" is a contradiction.
The core difference
The grammatical split is small and easily stated. The meaning problem underneath it is the one worth attention.
Grammar
- optimal — adjective only: the optimal temperature
- optimum — noun: the optimum lies near 40°C; adjective: the optimum temperature
Where an adjective is needed, both work and the choice is stylistic. Optimal is more common in technical writing; optimum feels slightly older.
The meaning worth guarding
An optimum is the best under given constraints — a single point, not a general compliment. That is why more optimal, most optimal and very optimal jar: they treat a superlative as though it had degrees. If you mean "better", write better.
Plural
The noun takes optima or optimums. Both are accepted; optima is usual in scientific writing.
Frequently asked questions
Is "more optimal" correct?
It is widely used but logically awkward, because an optimum is already the best available point. Better or nearer to optimal expresses the comparison without the contradiction.
Can optimum be used as an adjective?
Yes. Optimum works as both a noun and an adjective, which is the main way it differs from optimal.
What is the plural of optimum?
Optima or optimums. Optima is the usual form in scientific and technical writing.