Resolves to the first month an M3 iMac can be purchased or ordered. If it launches at midnight or is otherwise hard to tell, I may resolve multiple months.

From Mark Gurman's "Power On" newsletter on March 5, 2023

While development of the new iMacs — codenamed J433 and J434 — has reached a late stage, it’s not expected to go into mass production for at least three months. That means it won’t ship until the second half of the year at the earliest. Still, this is a great development for anyone disappointed that Apple’s all-in-one desktop hasn’t been updated in nearly two years. 

"Available for order" means anyone who didn't sign an NDA or other restrictive agreement can either buy one or place an order for one. I expect the apple.com website to resolve this market.

"M3 iMac" does require a new M3 line of Apple Silicon, so if it's released with a new or existing M2 chip it won't count.