Page 30, Column 3, Paragraph(s) 7, Core Rule Book
Flatline: A condition that results from the Runner being forced to trash more cards than he has in his grip, or from having a maximum hand size that is below zero at the end of his turn, and which causes the Runner to immediately lose the game.

Flatlining is the Runner's lose condition. The Runner flatlines if s/he takes more damage than s/he has cards in grip or if s/he has a negative maximum hand size at the end of his/her turn.

Page 20, Column 2, Paragraph(s) 9, Core Rule Book
If the Runner takes more damage than the number of cards in his grip, or if he has a maximum hand size of less than zero at the end of his turn, then he is flatlined and the Corporation wins the game.
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