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What is the difference in usage between maximum and maximal? When would you use one or the other?

Maximum can be a noun or an adjective:

This is the maximum it can be set to.
This is the maximum value.

whereas "maximal" is always an adjective:

This is the maximal value.

Is this correct? Is there more to it than that? When would you use one or the other as an adjective, or are they completely interchangeable?

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In your third example, maximum must be a typo for maximal. – Tsuyoshi Ito Aug 17 '11 at 12:22

2 Answers

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There is a subtle difference; maximum and minimum relate to absolute values — there is nothing higher than the maximum and nothing lower than the minimum. Maximal and minimal, however, can be more vague.

In "I want to buy this at minimal cost" and "this action carries a minimal risk", minimal means "very small" as opposed to "the lowest possible"; the same distinction is true of maximum and maximal.

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Why was this downvoted? Seems like a reasonable answer. – Clay Nichols Aug 17 '11 at 12:18
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Thanks @Clay, I'd love to know too! – Waggers Aug 17 '11 at 12:42
I don't believe this answer is correct. The usage you cite is simply a misunderstanding about 'maximum' etc., which are frequently used to mean 'very big' rather than their correct meaning which is 'the biggest'. – DJClayworth Aug 17 '11 at 14:29
Surely if words are frequently used to mean something then that becomes their correct meaning? Language isn't static. After all the question was about difference "in usage" not "in original meaning" – Waggers Aug 18 '11 at 7:53
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Waggers' answer does an excellent job of explaining the difference in the non-technical meaning. In some areas of mathematics (e.g., in maximal element, maximal matching), a maximal value (you can't use the with this meaning) is essentially a local maximum—it's a maximum value in its neighborhood; i.e., there are no small changes which will increase the value. See this Wikipedia article for a more technical description in relation to partially ordered sets.

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It seems to me that mathematical definition (a maximal value=the local maximum) effectively means the same as @Waggers "non-technical" one anyway. A maximal value would not be so called if it was easy to identify a higher one "nearby" (whatever "nearby" meant in context). – FumbleFingers Aug 17 '11 at 16:39
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@FumbleFingers: you're right ... it's essentially a mathematical adaptation of the non-technical definition. – Peter Shor Aug 17 '11 at 19:10
I have seen maximal used like in Peter's answer: In my algorithms class we used maximal to mean that we would have to backtrack to get a better(larger) answer. – Brian Mar 23 at 2:59

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