Expanding a bit: the more usual term is MCT -- "minimum connection time". There are actually multiple categories of MCTs:

* MCT D-D: Minimum connection time, from one domestic flight to another domestic flight.

* MCT D-I: Minimum connection time, from a domestic flight to an international flight.

* MCT I-D: Minimum connection time, from an international flight to a domestic flight.

* MCT I-I: Minimum connection time, from one international flight to another international flight.

These are set per airline per airport, and in many cases there are more fine-grained MCTs for flights depending on origin/destination or categorized by flight number. The airline will not let you purchase tickets for an itinerary which violates the relevant MCT, and if delays cause you to dip below the MCT you're possibly going to be rebooked (though no guarantee of it).

For completeness' sake, here's what the information looks like in raw form (this is the MCT for American Airlines at its hub in Charlotte):

STANDARD.D/D...D/I...I/D...I/I.
ONLINE .40 1.00 1.00 1.00
OFFLINE .40 1.00 1.00 1.00
** OR * ARE ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 2755 - 6099
EQP ALL - E70 PUERTO RICO - ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 2755 - 6099
EQP ALL - E70 VIRGIN ISLAND US - ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 2755 - 6099
EQP ALL - E75 PUERTO RICO - ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 2755 - 6099
EQP ALL - E75 VIRGIN ISLAND US - ALL
AA-AA ID SUP FLT 1 - 9099 - FLT 1 - 9099
CANADA - CANADA
AA-AA ID SUP FLT 1 - 9099 - FLT 1 - 9099
MEXICO - MEXICO
AA-AA ID .50 FLT 1 - 6099 - FLT 1 - 6099
IRELAND - ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 1 - 2754
PUERTO RICO - ALL
AA-AA ID .35 FLT 1 - 2754 - FLT 2755 - 6099
PUERTO RICO - ALL
AA-AA ID .30 FLT 1 - 2754 - FLT 1 - 2754
VIRGIN ISLAND US - ALL
AA-AA ID .35 FLT 1 - 2754 - FLT 2755 - 6099
VIRGIN ISLAND US - ALL
AA-AA ID 1.15 FLT 1 - 2754 - FLT 1 - 6099
GROUP3 - ALL
AA-AA ID 1.00 FLT 1 - 6099 - FLT 1 - 6099
**-AA ID 1.15 ALL - FLT 1 - 6099
**-AA ID SUP ALL - FLT 6100 - 9099
AA-** ID 1.15 FLT 1 - 6099 - ALL
AA-** ID SUP FLT 6100 - 9099 - ALL
USE T*GRPN FOR CITIES IN A GROUP. WHERE N IS THE GRP NUMBER.

The first few lines give the general rule ("ONLINE" means both flights on the same airline, "OFFLINE" means connecting to a flight of a different airline), and after that come exceptions. For example:

AA-AA ID SUP FLT 1 - 9099 - FLT 1 - 9099
CANADA - CANADA

"AA-AA" means from American Airlines to American Airlines. "SUP" means "suppressed": the connection is invalid. "FLT" gives a block of flight numbers the exception applies to (1 through 9099). "CANADA - CANADA" gives the origin of the inbound and destination of the outbound flights. So this is a fancy way of saying American forbids booking a flight from somewhere in Canada, to somewhere in Canada, connecting in Charlotte, on a flight numbered 1 though 9099, regardless of the amount of time of the connection. Other codes like "EQP" refer to the type of aircraft ("N" for narrowbody, "W" for widebody, "ALL" for all types).

You can spot other fun things in there, like the shorter time to connect on a flight from Ireland to a domestic flight; AA flies to Charlotte from Dublin, and Dublin has US Customs preclearance facilities, so passengers arriving from Dublin don't have to clear customs in Charlotte and can make a tighter connection.