My iMac connects to the internet (and my local network) via WiFi. Is it possible for me to connect another device via ethernet cable to my iMac to access the internet from that device?
You sure can! Open System Preferences, then go to Sharing and select Internet Sharing. Change "Share your connection from:" to Wi-Fi, and then in the box below it select Ethernet.
Sharing your source Internet using WiFi to your Ethernet output.