For most people, the phrase "computer terrorism" has a rather arcane sound - a little-understood activity that has no relation to real life. Yet that phrase is what Pentagon experts are beginning to use to describe the breaking into its computer networks.

Twice in the past month, "intruders" have penetrated Pentagon systems and caused problems, delays, and even damage to some computer files. So far, there is no reason to suspect more than sophisticated pranksters. But with so much information now in electronic files and so much vital work being done by computers, the potential for harm is very great.While young computer "hackers" are likely to blame, this is not a matter of fun and games. When Pentagon files are penetrated, the repair work usually requires shutting down part of the system to clean out electronic "junk" and the rewriting of various programs to handle the problem. The cost can easily run into millions of dollars.

If a serious computer terrorist decided to wreak havoc in the nation's military computers, the problem could be enormous. In recognition of that fact, the U.S. last month set up an electronic counter-terrorist force. Instead of armed commandoes, this group is made up of software engineers on call to deal with invasion of government computer systems.

Finding computer intruders is not easy because computer systems are tied together nationwide. The latest break-in involved the Milnet system. Operating from a computer in an unknown location, the intruder leap-frogged through several university computers in the U.S. and Canada and into the headquarters of a high-tech contractor. There may even be some connection with computers in Great Britain, so national borders are not defense.

The motivation is hard to understand. Who would jeopardize top-secret files, millions of dollars worth of computer programs, and impact on the nation's military computer system, just for fun?

The worry among Pentagon experts is that the time may come when more than pranksters are at fault - that real computer terrorism may be practiced with very significant damage. With every advance in science and technology, there is a corresponding amount of new trouble.