Tj max

[looney]

What exactly is TJ max, in rgerads to temps?

[peterb]

It is the maximum junction temperature that the CPU's core is allowed to achieve. It is specified in the processor's data sheet, and in the case of Intel processors (I haven't looked at AMDs) the processor starts throttling at an unspecified point before that temperature is reached to protect itself from damage. At Tj max, the case temperature will be (typically) about 20C lower (according to intel's spec sheets)

[Thorsson]

Also at an unspecified point above TJMax (but ~20C) the processor will shut down.

It's not always specified in the processor's data sheet. The reason that that the various CPU temp measures have had problems, and why they tend to disagree, is that Intel hasn't specified TJMax in many cases, so the authors estimated it. Moreover Intel has changed TJMax on the same chip with different fabrications.

Do a Google on Real Temp vs CoreTemp and you should find some very interesting discussions on this topic.