  Precision pulsar timing at the level of tens to hundreds of nanoseconds
allows detection of nanohertz gravitational waves (GWs) from supermassive
binary black holes (SMBBHs) at the cores of merging galaxies and, potentially,
from exotic sources such as cosmic strings. The same timing data used for GW
astronomy also yield precision masses of neutron stars orbiting other compact
objects, constraints on the equation of state of nuclear matter, and precision
tests of General Relativity, the Strong Equivalence Principle, and alternative
theories of gravity. Timing can also lead to stringent constraints on the
photon mass and on changes in fundamental constants and could reveal low mass
objects (rogue planets, dark matter clumps) that traverse pulsar lines of
sight. Data sets also allow modeling of the density, magnetic field, and
turbulence in the interstellar plasma. Roughly 100 millisecond pulsars (MSPs)
are currently being timed at $\sim$GHz frequencies using the largest radio
telescopes in the world. These data sets currently represent ~1000 pulsar-years
and will increase dramatically over the next decade. These topics are presented
as a program of key science with flowdown technical requirements for achieving
the science.
