Open Content, to me, is that body of creative works, both digital and physical, that are available to all to use, modify, and distribute. Typically this includes text, images, video and audio recordings. I distinguish it from Open Source software in that it is not executable as a program by a computer.
That's a weak distinction, I know. Very few programs of any significance in this day and age don't incorporate some significant text or images, and many include video and audio as well. And a computer can be designed to execute in some way any arbitrary data stream, no matter what the original purpose of the work. Needless to say, this is a thorny issue.
I tend to be a pragmatist about which words we use to describe concepts like this. Language is about communication, and if I can communicate the idea described above to use using the name Open Content, great. I also use interchangeably Free Content and occasionally OFLACC, an acronym I made up for Open/Free/Libre Art/Content/Culture. If you need me to say it to you in some other way, I'm happy to do so. It's the signified, not the signifier, that matters here.