Is Android a 32-bit or 64-bit OS?

The title is pretty much self-explanatory: is Android a 32- or 64- bit OS? I assume that it is one or another and not both, as that would force both 32- and 64-bit binaries to be hosted on Google Play. I know that this is a simple question, and that other people have been asking it, but for the life of me I have not been able to find an answer on the web, and there is certainly not one on this SE.