**Uncountable Infinity**
An order of infinity that is larger than the number of natural
numbers. The number of real numbers is uncountably infinite.

**Universal Approximation**
Having the ability to approximate any function to an arbitrary
degree of accuracy. Neural networks are universal approximators.

**Universal Computation**
Capable of computing anything that can in principle be computed; being
equivalent in computing power to a Turing machine, the
lambda calculus, or a Post production system.

**Universal Computer**
A computer that is capable of universal computation, which means
that given a description of any other computer or program and
some data, it can perfectly emulate this second computer or
program. Strictly speaking, home PCs are not universal computers
because they have only a finite amount of memory. However, in
practice, this is usually ignored.

**Unstable**
Having a basin of attraction that is 0 in size; being such
that the slightest perturbation will forever change the state of
a system. A pencil balanced on its point is unstable.