This is a Sigma name for one of their lens lines. It is not a general purpose term and really only has the marketing meaning Sigma ascribes to it. From the Sigma website, that definition is:

Engineered for today’s photographer who wants to do it all, our Contemporary line combines superior optical performance and compactness for high-performance lenses that cover a wide range of needs.

Other Sigma lines are Art and Sport. The marketing copy for all three lines is kind of ... fluffy, but at least those two give an idea of what to expect, while "Contemporary" seems like it could mean just about anything.

Sigma offers a 150-600mm lens in both the Sport and Contemporary series. Comparing the two makes the distinction between lines clear. Both lenses are variable-aperture at f/5 through f/6.3, but the construction is very different:

• The Sport (or just "S") version uses 24 elements in 16 groups, while the Contemporary ("C") version has just 20 in 14 groups. A more complicated design implies better correction of optical artifacts — but add size, weight, and cost.
• It's not just more elements, either. The glass in the S version is literally bigger. The S version takes a 105mm filter, but the C version has a more compact front element, with a 95mm filter size.
• And not just that — those large lenses in the front group in the S version are made of Sigma's most expensive "FLD" glass, whereas in the C version, that's only used for the much smaller rear element.
• Both lenses feature water- and oil-repellent coatings on the front element, but the S version has that on the rear element as well — and while the S version claims "dust and splash-proof construction", the C version just says "dust proof and splash proof mount" [emphasis added].
• All of this means that the S version weighs 2860g, while the C version is just 1930g — only ⅔ the weight.
• And, finally, the S version lists for $1999, and the C version less than half at $989. What's more, this is the cheapest S lens, and $989 is by far the most expensive C lens.

The 30mm f/1.4 Art and Contemporary lenses provide another example. Here, we have 9 elements in 8 groups vs. 9 in 7; a filter size of 62mm vs. 52mm; weight of 435g vs. 265g, and $499 vs. $339.

Overall, I think it's safe to say:

• Art lenses: priority on optical performance and rendering
• Sport lenses: high-end telephotos and zooms, sacrificing size, weight, and cost for optical performance and a more solid build
• Contemporary lenses: a nice way of saying "this is the budget line filling what a lot of people really want but would be put off from if we actually called it 'budget', and also they might demand that they be even cheaper".