Wordnet 3.0
NOUN (1)
1. 
 electronic warfare undertaken under direct control of an operational commander to locate sources of radiated electromagnetic energy for the purpose of immediate threat recognition; 
[syn: electronic warfare-support measures, ESM]
WordNet (r) 3.0 (2006):
ESM
    n 1: electronic warfare undertaken under direct control of an
         operational commander to locate sources of radiated
         electromagnetic energy for the purpose of immediate threat
         recognition [syn: electronic warfare-support measures,
         ESM]
V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):
ESM
       External Storage Module (Sun)