# Meaning of “-oid” in mathematical concepts?

Some mathematical concepts are ended with "-oid", such as Matroid, greedoid, groupoid. What does that mean? Do these concepts share something in common? Thanks!

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i know humanoid means a robot –  Santosh Linkha Jan 11 '13 at 0:10
It's not specifically math terminology. See here. –  David Mitra Jan 11 '13 at 0:11
@DavidMitra: Groupoid is like group, and matroid is like ..., and Greedoid is like ...? –  Tim Jan 11 '13 at 0:13
Matrix and, um, Greedo from Star Wars? –  David Mitra Jan 11 '13 at 0:13
Hopefully the string hemorrh never makes into the mathematical lexicon. –  copper.hat Jan 11 '13 at 0:22

Means "bed", "incomplet","deficient" or "weak". For exemple: Grupoid is a algebric struture "deficient" that is quase a Group.

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Is it like quasi-, semi-, pseudo-‌​? –  Tim Jan 11 '13 at 0:17
@Elias, a groupoid is a category in which all arrows are iso. This includes the category $\bf Group$ , which is a monoid (ie, one-object) groupoid. See: ncatlab.org/nlab/show/groupoid –  alancalvitti Jan 11 '13 at 0:25
@Elias, - just wanted to clarify, groupoids are generalizations of groups, but in what way are they deficient or quasi? (There's an interesting if cryptic remark in Lawvere & Rosebrugh's Set Theory for Mathematics to the effect that most of what we know about groups carries over to groupoids.) –  alancalvitti Jan 11 '13 at 0:39
Sounds a lot like "-ish." Such a shame we didn't end up with matrish or groupish. –  Jesse Madnick Jan 11 '13 at 0:40
@JesseMadnick, Good +1. –  alancalvitti Jan 11 '13 at 0:57

If you have a "center" of an object, it means "equidistant" from the edges (spacially).

If you have a centroid, it means the weighted "center" of an object, where the weightS are something like the density of the material.

Here the "oid" is a reference to the "weighted" (e.g. center) property, not the literal one.

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