QUESTION:
What is an "SEU"?

ANSWER from David Dubov:
SEU = Single Event Upset (also called SEE or Single Event Effect).

An SEU occurs when a high energy ionized particle (say an ionized oxygen
atom traveling at 50% the speed of light) which is generated by either a
magnetic storm on the Sun or a galactic cosmic ray (the source of these
particles is still somewhat unknown), hits the spacecraft, goes right
through the structure and happens to hit an electronic memory cell (a
"bit"). It usually doesn't do much permanent damage to the cell, but the
event could easily flip the bit from a binary "1" to a "0" (or vice versa)
thereby confusing the on-board software. Electronics used on spacecraft are
usually as expensive as they are due to the fact they often must be
specially designed and manufactured to be "hardened" to SEUs and other types
of non-ionizing radiation effects.