Um

Umber
A brown pigment comprised of hydrated ferric and manganese oxides with variable proportions of earthy matter.
Umbilical Tube
A tube that connects a growing embryo to the placenta.
Umbra
The darkest part of a shadow where no light from the source reaches.
Umkehr Effect
An anomaly of the relative zenith intensities of scattered sunlight at certain wavelengths in the ultraviolet as the sun approaches the horizon, due to the presence of the ozone layer.