Google will be releasing a new Android OS at Google I/O in just a few days, but the name will be a letter not the real name. It will likely just be called Android M until it's released, but Google has an internal code name just as it did for L and K. It's called Macadamia Nut Cookie (MNC), a name which is already being referenced in AOSP.
There have been some small references about the MNC name for a while now, but only recently has it started popping up in AOSP as "mnc-dev." We'll probably never see an OS version that uses this name or we might.
In the past, Google has used both LMP (Lemon Meringue Pie) and KLP (Key Lime Pie) as internal code names for upcoming versions of Android (ex. lmp-preview-release in AOSP). Those names were abandoned for the official Lollipop and KitKat upon release. We have every reason to think the same will be true for MNC, but in the meantime you can see a few places in AOSP where mnc-dev is mentioned (here and here).
There have been some small references about the MNC name for a while now, but only recently has it started popping up in AOSP as "mnc-dev." We'll probably never see an OS version that uses this name or we might.
In the past, Google has used both LMP (Lemon Meringue Pie) and KLP (Key Lime Pie) as internal code names for upcoming versions of Android (ex. lmp-preview-release in AOSP). Those names were abandoned for the official Lollipop and KitKat upon release. We have every reason to think the same will be true for MNC, but in the meantime you can see a few places in AOSP where mnc-dev is mentioned (here and here).
This might provide some clues as to what's happening with Android M going forward. Don't expect Google to acknowledge this name in public, though.