I've read the yield strength of a material can be improved either by age hardening or mill hardening treatment. Age hardening is primarily a function of time and temperature with some environmental and chemical factors. But what is mill hardening? Hardening a material by ... milling something like a hole through it?
Mill hardening is a catch-all term for any set of processes that produce a standard alloy with defined properties from the metal mill. It is used when talking about treatments by a material vendor that a workpiece will have been received with. In the shop you may apply further treatments to different sections, but the "mill hardening" would refer to the as-delivered material.
If you bought mild steel and then forged and treated it yourself you would call it age hardened. If you bought engineered steel with controlled properties it would be received as "mill hardened" to that spec. The term can be thought of as interchangeable with "age hardening", but specifically means to refer to treatments applied by the material vendor before the material reaches the shop.
It is also frequently seen when vendors wish to hide the specifics of a proprietary treatment - they will say it is "mill hardened" with a proprietary process, so you know the material you are buying has had heat treatment and work of some sort, and that it meets the advertised specifications, but you don't get the recipe so you can't reproduce it yourself.