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I was wondering about the meaning of "pseudo-", "quasi-" "semi-" and possibly other related prefixes, in general cases.

Particularly, in engineering and science, there are quite a few terms named with these prefixes. For example, "quasi-integrable", "pseudo-Newton method", "semigroup". But I never get to understand their differences in usage.

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    Just to add some color to the conversation... a hemidemisemiquaver is a 1/64 note.
    – oosterwal
    Apr 5, 2011 at 14:30
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    Musicians who lack rhythm and timing play their 1/64 notes as pseudoquasisemiquavers.
    – oosterwal
    Apr 5, 2011 at 19:48
  • Which prefix is preferred if one object has some of the defining characteristics of another but may or may not have all or be of the same (a priori specified) class? For example, two polynomials may have the same number of roots and in the same location but not be equal functions.
    – user02138
    Jul 16, 2011 at 22:34
  • any of them (see @Mitch's answer about math terms)
    – éclairevoyant
    Nov 24, 2018 at 15:28
  • There is also the "-oid" suffix used in a similar way, e.g. "asteroid" = "like a star", "cuboid" = "like a cube", etc.
    – Dai
    May 2 at 7:05

4 Answers 4

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Pseudo- comes from the Greek for false/lie and refers particularly to something not genuine.

Quasi- comes from the Latin for almost and refers to something that is almost something else.

Semi- is the Latin for half and refers to something that is half-something else

In late usage, however, all three have been used to mean sort-of-

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As others have said, pseudo = false, quasi = almost, semi = half.

In engineering and science, these tend to have fairly technical meanings. That is, a pseudopod is something that looks like a foot but is not, semiannually means twice a year (and I can't really think of an accepted repeatable term that begins with 'quasi').

In mathematics, however, all bets are off. A semigroup is like a group and has fewer defining properties than a group, but in no way is it anything like half of a group. Similarly a quasigroup. (this is a general notice about mathematical terminology, the inventors of the terminology usually have some metaphor in mind when the technical term is chosen, but that metaphor hardly ever translates to anything meaningful).

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    Quasar: Quasi-stellar radio source (quasi-stellar, because it seems like a star, but is infact a galactic nucleus)
    – Dancrumb
    Apr 5, 2011 at 1:59
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    @Mitch, so, all you've found are quasi-words? ;)
    – msanford
    Apr 5, 2011 at 2:11
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    And pseudocode is stuff that looks like code but isn't.
    – Jon Purdy
    Apr 5, 2011 at 7:10
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    @Mitch: my favourite being: Quango - quasi-autonomous non-governmental organisation. Not so much a nonce word, more a nonsense word.
    – Matt E. Эллен
    Apr 5, 2011 at 8:28
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    Actually quasicode is a closer description for pseudocode than pseudocode.
    – SWeko
    Apr 5, 2011 at 10:25
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  • Quasi - almost
  • Pseudo - fake or impersonating
  • Semi - half
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In civil engineering the term quasi-permanent is used for actions (loadings). It has more the meaning of equivalent that almost.

In very simple words, we suppose that the action is permanent although it's true nature is quite different. This in practice means that we load the structure with the equivalent of a permanent load.

What permanent means in civil-engineering context is another question!

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