In the minds of advanced medical thinkers in the Guild of Doctors, a species of lifeform which is so incredibly small it cannot be seen with the naked eye, and which is held responsible for causing illnesses and infections.
The term was later used by the Smoking Gnu to describe packets of pre-programmed code which, if uploaded to the Grand Trunk clacks network, would make it very ill indeed, while showing no apparent cause.
Obviously parodies Roundworld's viruses and computer viruses. In Interesting Times, Rincewind mentions "great big seals vif teece ike iff", apparently mimicking a Roundworld walrus. In other words, Roundworld walruses are just called "seals" on Discworld.
"Walrus" sometimes means "walrus", however; for instance in The Last Hero Cohen the Barbarian demonstrates his foresight and leadership by yelling, "Look out, we're going to be attacked by a giant walrus!" just before this, in fact, happens. The Silver Horde are able to feed on walrus meat for some time thereafter.