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Coming from a technical background I'm slightly confused. What is the difference between simultaneously and concurrently? How do we use these words?

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closed as general reference by JSBձոգչ, Mahnax, Daniel, FumbleFingers, jwpat7 Mar 15 '12 at 15:52

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I just looked these up in an online dictionary; this is what I found:

concurrently: overlapping in duration

simultaneously: at the same instant

Both of them mean "at the same time," and are almost interchangeable.

That said, if I had to point out a slight difference, I'd say that concurrently occurs over a longer time, and is a bit less synchronized than simultaneously.

For example:

The two processors were running concurrently, then they crashed simultaneously.

In that example, I wouldn't swap the two words in question.

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Wow great explanation and example. Thank you. –  Jackie Chan Mar 15 '12 at 3:52
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In addition, concurrent has a few additional meanings, most of which amount to "in agreement," as in "two people working concurrently." That is to say, they're working towards the same goal. The other meaning of concurrent is related to geometry, where you might say two lines were concurrent: they're going to intersect at a common point.

EDIT:

[I don't have enough rep to comment beneath this yet, but I'd like to say I think you guys should stop commenting. I don't think you're adding anything anymore.]

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In the geometry context, here are 477 definitions, most of which say you need at least three lines intersecting at the same point in order to call them "concurrent". It would be pretty trivial to apply the term to just two lines, since they will always intersect unless they're parallel –  FumbleFingers Mar 15 '12 at 4:27
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@FumbleFingers: unless the two lines are in 3-dimensional space, and not in the same plane... –  J.R. Mar 15 '12 at 10:16
    
@J.R. I'm no mathematician, but Googling the two quoted strings "concurrent lines" "3-d space" gives only 42 results in total, many of which are about projecting 3-d lines onto a 2-d surface in order to manifest "concurrent lines". Which might even suggest that mathematicians avoid the term outside of plane geometry. –  FumbleFingers Mar 15 '12 at 13:42
    
@FumbleFingers - Your several contentions about concurrent lines are incorrect. See for example wikipedia, which says "In projective geometry, in two dimensions concurrency is the dual of collinearity; in three dimensions, concurrency is the dual of coplanarity." –  jwpat7 Mar 15 '12 at 16:00
    
@jwpat7: I'm not making a "contention", I'm simply presenting the usage statistics from written sources. In such a restricted usage it doesn't really mean anything to quote one "authoritative" definition or another - all that matters is whether people actually use the terminology in particular ways. It would appear that, Wikipedia notwithstanding, they don't often use it in a 3-d context - plus there are those hundreds of written instances explicitly saying "concurrent" only applies with at least 3 lines. Their words, not mine. –  FumbleFingers Mar 15 '12 at 16:21
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