A new feature for eBooks is "touch a word to get a definition" in the OMC app. To learn more about how to use this feature along with other features like taking notes when using OverDrive Read (not the app) see the OverDrive Help article How to take notes, highlight text, and use the dictionary in an OverDrive Read eBook.
Touch to Define (offline- not on the Internet) is ONLY a feature on the iOS App. Android devices MUST be connected to the Internet for that feature to work.
Adding "Touch to Define Offline" to the Android App is on OverDrive's Wish List, but it's not in development at this time, and there's no ETA for that.