Net damage is one of the three types of damage that a Runner can suffer. Net damage is mechanically identical to meat damage, requiring the Runner to trash one card from grip at random, but is caused and prevented by different cards from meat damage. If the Runner takes too much damage, s/he can flatline.

Page 20, Column 2, Paragraph(s) 5, Core Rule Book
Net Damage: The Runner randomly trashes one card from his grip for each point of net damage done to him.
Page 30, Column 3, Paragraph(s) 1, Core Rule Book
Damage, Net: A unit of damage that requires the Runner to trash one card from his grip at random.
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