Homogeneous means (1) of the same or similar nature, and (2) uniform in structure or composition. Its corresponding noun is homogeneity. Homogenous, whose corresponding noun is homogeny, is a little-used biological term whose old sense has mostly been lost. Today, it’s primarily a variant of homogeneous in general usage, though it still has uses in science, where spelling it any other way would be considered an error.
Though some careful nonscientific writers continue to try to keep the words separate, it is a lost cause in popular usage. And the change is not new; people have been using homogenous in place of homogeneous throughout the English-speaking world for at least a century, and homogenous is now several times more common than homogeneous outside scientific writing, so trying to preserve the distinction is probably a losing battle.
Though homogenous is now more common than homogeneous, the latter still appears fairly often—for example:
The Arab world is not at all homogeneous and responses to unrest will play out very differently in each country. [Telegraph]
Complex fluids can be considered homogeneous at the macroscopic (or bulk) scale. [Physics]
But unlike the homogeneous Apple iPhones, there are many different flavors of Droid. [GCN]
Finding examples of homogenous used in its traditional sense is almost impossible. Searching the web, we mostly find examples like these:
Kiss Each Other Clean is much more focused and homogenous, but there’s still a lingering sense of abundant inspiration, eager to carry the songs off to different lairs. [The Independent]
Research has also proven that well-led diverse groups are better at problem solving and homogenous teams run the risk of “groupthink.” [Huffington Post]
After getting some flack for a rather homogenous cover of white actresses last year, Vanity Fair’s new Hollywood issue is mixing things up. [Gawker]