A word used to talk about something on a net both without letting the enemy know what it is and to communicate an exact point exactly. For example, SUNRAY always means 'the boss' but if Ze Germans hear it over the net they will think 'vot?'
Phonetic codeword systems are unambiguous words that can be pronounced and understood by those who transmit and receive voice messages by radio or telephone regardless of their native language, especially when the safety of navigation or persons is essential. Normally, uncommon and highly differentiable words are selected to represent otherwise ambiguous letters, numbers or words in order to clarify potentially confusing communication. Ideally, such codewords are uniformly pronounced by all speakers.
One such system in practical use is the NATO Phonetic Alphabet which principally addresses the homophony of distinct letter/number names across languages and the near-homophony of many letter/number names within single languages. For example, it is difficult to distinguish between the number 3 and the letters b, c, d, e, g, p, t, v and z in spoken English. The NATO phonetic alphabet replaces these common words with TREE (Spelled as: TANGO-ROMEO-ECHO-ECHO), BRAVO, CHARLIE, DELTA, ECHO, GOLF, PAPA, TANGO, VICTOR and ZULU respectively.
Operational Codewords are chosen to disguise the likely intent behind future, past and present operations. Unless you're American in which case 'the operation to invade Iraq and liberate the people' is not OPERATION TELIC but OPERATION IRAQI FREEDOM, which must have taken Saddam's intelligence corps about five minutes to work out.