Some of these terms are used a bit loosely and it would be helpful if you either cited, or quoted from, your reference so that the usage is seen in context.
There are at least three applications of the term which you might come across ..
(a) A simple mechanical limitation restricting thrust output so as to achieve a maximum power rating as might be seen in a turboprop installation
(b) A manufacturer election to downgrade the maximum thrust output of the engine, ie use a bigger engine where a smaller one would do the job .. often to improve the higher OAT performance
(c) If you look at a typical thrust carpet in a Flight Manual performance chart, the basic appearance of an individual altitude line is of two straight-ish lines which meet at a point somewhere near the higher OAT end. (The line might well be a little more complex than this .. but this is the simplest presentation.)
(i) the more steeply sloping line to the right (higher OAT) of the intersection usually represents the thrust (fuel) reduction necessary to remain within engine temperature limits.
(ii) the intersection usually is called something like the "thrust break temperature"
(iii) the shallower sloped line to the left (lower OAT) of the intersection usually represents a mechanical limit within the engine which precludes operation under temperature limited conditions. This section of the operational envelope often is referred to by the term "flat-rated" which is a reference to the comparatively small variation in thrust within this OAT range .. the line itself is not all that far from being horizontal.
[ 18 February 2002: Message edited by: john_tullamarine ]</p>