Ambient, the environment here and now. Whatever
touches the device. Ambient air however has other
key attributes such as flow.
A device's attributes (as listed) are specified between
min and max temperatures. Case temp is used in the
HiRel / military parts worlds because it can be exactly
forced and measured. Which is key since you've got
delta limits besides.
Anyway, between min and max temp as specified
(perhaps a JEDEC callout, you'd have to do the digging
and lots of companies don't have very good info in
the "first layer" product documentation, unless you get
into the more-testing grades) the performance and reliability
limits in the tables are "guaranteed"* to apply. Outside
they may or may not, it's on you.
* Ha, ha. Read the fine print.
Ambient temperature 0 - 70C is a common commercial
range and you should realize that on many low cost
commercial parts there is no testing done after packaging.
Just probe and pick to wafer map all robo-style.
A higher grade might see actual package test.