Does DC adapter mean the device converts DC to ac or ac to DC?

An AC adapter adapts the device that one wants to power up to the AC line. That means it converts the AC line voltage to something that can be used by the device in question. An AC adapter converts AC to DC. (In contrast, a device that runs off DC and makes AC out of it is called an inverter. An example of an inverter is the device that plugs into a vehicle's cigarette lighter and has an outlet on the other end that will permit someone to plug something like a kitchen blender into it. Great for tailgating! Don't run your vehicle's battery down....) There are times when an AC adapter is the little plug-in-the-wall thingie that converts the AC line voltage to some lesser value of AC. There are a few devices out there (usually older ones) that used their AC adapters to step down the wall voltage (115 volts AC) to 24 volts AC, 18 volts AC, 9 volts AC, or anything like that. Some new sprinkler control timers use an adapter that is an AC -to-AC step down converter. By looking at the information on the adapter, the user can see what the primary voltage is (and it's almost invariably 115 volts AC or 120 volts AC - same, same for this application) and what the secondary voltage is. As stated, the secondary is almost always DC, and if it is, the polarity is almost always indicated as well. That's so a user can tell if that little round connector has positive on the inside and negative on the outside or the other way round. (It makes a lot of difference to the device on which it is used, as one might guess.)