Page 31, Column 1, Paragraph(s) 15, Core Rule Book
Memory Unit (MU): A unit of space available to the Runner to install programs. The Runner begins the game with four memory units.

Memory units (MU or MemoryUnit-1) are a special kind of resource in Android: Netrunner that place a constant restriction on how many programs the Runner can have installed at any given time. The Runner starts the game with 4MU, but this number can be increased or decreased by various cards and effects. Each program has a memory cost, which counts against the Runner's available MU. If the Runner ever has a combined total memory cost of programs greater than his/her available memory units, programs must be trashed.

Page 15, Column 1, Paragraph(s) 2-3, Core Rule Book
The Runner begins the game with four MU, though certain card effects can increase or decrease this value.

If the MU cost of the Runner's installed programs ever exceed his available MU, he must trash his installed programs until he is no longer exceeding his available MU.

Page 22, Column 2, Paragraph(s) 7, Core Rule Book
MemoryUnit-1: This symbol stands for memory unit. It always appears with a quantity, such as +MemoryUnit-2, which means "plus 2 memory units."
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