It would mean the meat has only had about half the required cooking. What is "required" may be a matter of opinion.
Waiter! This steak is only half cooked. I ordered "well done steak". Take it back to the chef and give it another 10 minutes on the grill!
When the chicken is half cooked, take it out the oven and baste it with the juices. Then return it to the oven for the remaining time.
Meat that is intentionally left pink in the middle is called "rare" (and there are variations: medium rare, very rare, and well done (=not rare at all)
In your images, the tuna steak would be better described as "medium rare tuna steak on a plate". And the photo-edited image of a steak that was completely uncooked on the left and completely cooked on the right is "half cooked and half uncooked steak" but the actual object is very unlikely to to exist (who cooks a steak only on the right??)