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I've seen constraint and restriction used quite often in scientific literature.

For example,

This algorithm needs to be further improved due to severe space constraint and restriction.

I simply can't figure out the subtle difference between constraint and restriction. They seem to be circularly defined according to each other. I assume that they must be different in some way. Otherwise, they wouldn't have been used together so often. If they were really close enough in meaning, why not just drop one and use the other? Or are they used in combination to place an emphasis?

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What does the dictionary tell you? – coleopterist Nov 29 '12 at 5:03
@coleopterist If you did check the dictionaries and found it, please do let us know for our information, thanks a ton. – Kris Nov 29 '12 at 7:48
Dictionaries don't seem to help much. They don't say that one is always or usually ex post facto and the other ab initio. They just ramble on about the various meanings. – user21497 Nov 29 '12 at 9:07
What is this example from? Please cite source, preferably with a link, so we can consider the context. Or provide an alternate example. – MετάEd Dec 3 '12 at 17:44

2 Answers 2

up vote 3 down vote accepted

You may find the entries within's “Specialty Definition” sections for these words useful. For example, two of the several entries there include:

[Noun] Limitation; confinement within bounds. This is to have the same restriction as all other recreations. Restriction of words is the limitation of their signification in a particular manner or degree. Source: Webster's 1828 American Dictionary. [Specialty Definition: restriction]

[Noun] Irresistible force, or its effect; any force, or power, physical or moral, which compels to act or to forbear action, or which urges so strongly as to produce its effect upon the body or mind; compulsion; restraint; confinement. Not by constraint, but by my choice, I came. Feed the flock of God, taking the oversight thereof, not by constraint, but willingly. 1 Peter 5.. Source: Webster's 1828 American Dictionary. [Specialty Definition: constraint]

I generally think of constraints as restrictions that are intrinsic to a problem or situation (thus, more of an ab initio nature than ex post facto) and of restrictions as constraints that are imposed upon a problem or situation (thus, more of an ex post facto nature than ab initio). That is, I think Bill Franke has it backwards.

For example, an ordinary linear-programming problem is subject to hyperplane constraints; for example, the amount of X in some product, plus the amount of Y, not to exceed some limit Z. If on top of such constraints it were dictated that solutions must have integer values, one would speak of restricting the solution, rather than constraining it.

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Well, I did say that it was a purely personal distinction "based on how I see the words used and how I would use them". My first sentence comes from a paper I just edited. For the second, I would not use "the length constraint" for a word limit, would you? I don't think specialty definitions from any field are applicable because they are restricted to the field that stipulates the definitions. But, then, this is what usage questions are all about. They aren't about absolutes. – user21497 Nov 29 '12 at 9:00
Here's a circular definition: "budget restriction : The constraint of a budget. This option is used to avoid exceeding a defined budget." Alaska's Dept of Accounting sees "restrictions" as ex post facto & ab initio. This article sees restrictions as ab init. The interpretations and uses of these two words varies so broadly that appealing to a dictionary seems pointless. They both mean red lines. – user21497 Nov 29 '12 at 9:20

According to the Oxford dictionary:
Restriction, constraint, restraint, or limitation?
These are all things that limit what you can do. A restriction is a rule or law that is made by someone in authority. A constraint is something that exists rather than something that is made, although it may exist as a result of someone's decision. A restraint is also something that exists: it can exist outside yourself, as the result of someone else's decision; but it can also exist inside you, as a fear of what other people may think or as your own feeling about what is acceptable:moral/social/cultural restraints. A limitation is more general and can be a rule that someone makes or a fact or condition that exists.

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