| An IR Plume (Infrared Plume) is a Bad Thing for modern combatants who wish to remain concealed. Essentially, it's any sort of large emission of gas or water which is hotter than its surrounding medium, or it can be used to refer to the emission of infrared radiation directly from a heat source. In any case, it usually means that an enemy with IR detection gear can spot the plume, and possibly determine the emitter's location and intent.
Some examples include: opening the hatch on a vehicle with internal heating systems while the ambient temperature is low; firing an anti-tank weapon such as a MILAN or TOW, or (the ultimate) launching an ICBM. In each of these cases, from smallest to largest, the action produces an easily detectable plume of gas which radiates in the IR range. | 
