The abundance of a chemical element measures how relatively common the element is, or how much of the element there is by comparison to all other elements. Abundance may be variously measured by the mass-fraction (the same as weight fraction), or mole-fraction (fraction of atoms, or sometimes fraction of molecules, in gases), or by volume fraction. Measurement by volume-fraction is a common abundance measure in mixed gases such as atmospheres, which is close to molecular mole-fraction for ideal gas mixtures (i.e., gas mixtures at relatively low densities and pressures).
For example, the mass-fraction abundance of oxygen in water is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole-fraction abundance of oxygen in water is only 33% because only 1 atom in 3 in water is an oxygen atom. In the universe as a whole, and in the atmospheres of gas-giant planets such as Jupiter, the mass-fraction abundances of hydrogen and helium are about 74% and 23-25% respectively, while the (atomic) mole-fractions of these elements are closer to 92% and 8%. However, since hydrogen is diatomic while helium is not in the conditions of Jupiter's outer atmosphere, the molecular mole-fraction (fraction of total gas molecules, or fraction of atmosphere by volume) of hydrogen in the outer atmosphere of Jupiter is about 86%, and for helium, 13%.
Most abundances in this article are given as mass-fraction abundances.

Abundance of elements in the Universe
See also: Earth#Chemical composition
The Earth formed from the same cloud of matter that formed the Sun, but the planets acquired different compositions during the formation and evolution of the solar system. The history of Earth caused parts of this planet to have differing concentrations of the elements.

Abundance of elements in Earth's crust
"Rare" earth elements is a historical misnomer; persistence of the term reflects unfamiliarity rather than true rarity. The more abundant rare earth elements are each similar in crustal concentration to commonplace industrial metals such as chromium, nickel, copper, zinc, molybdenum, tin, tungsten, or lead. Even the two least abundant rare earth elements (Tm, Lu) are nearly 200 times more common than gold. However, in contrast to ordinary base and precious metals, rare earth elements have very little tendency to become concentrated in exploitable ore deposits. Consequently, most of the world's supply of rare earth elements comes from only a handful of sources.
Differences in abundances of individual rare earth elements in the upper continental crust of Earth represent the superposition of two effects, one nuclear and one geochemical. First, rare earth elements with even atomic numbers (₅₈Ce, ₆₀Nd, ...) have greater cosmic and terrestrial abundances than adjacent rare earth elements with odd atomic numbers (₅₇La, ₅₉Pr, ...). Second, the lighter rare earth elements are more incompatible (because they have larger ionic radii) and therefore more strongly concentrated in the continental crust than the heavier rare earth elements. In most rare earth deposits, the first four rare earth elements - La, Ce, Pr, and Nd - constitute 80 to 99% of the total.

"Rare earth" element abundances
See sea water for abundance of elements in the ocean, but note that that list is by mass - a list by molarity (mole-fraction) would look very different, especially as regards the first 4 elements; specifically, hydrogen would comprise well over half of such a list.

Ocean
The order of elements by volume-fraction (which is approximately molecular mole-fraction) in the atmosphere is nitrogen (78.1%), oxygen (20.9%), argon (0.96%), followed by (in uncertain order) carbon and hydrogen. The carbon and hydrogen concentrations are variable for a number of reasons, including human activity. Sulfur, phosphorus, and all other elements are present in significantly lower proportions.
According to the above graphic, argon, a significant if not major component of the atmosphere, does not appear in the crust at all.

Human body

Abundances of the elements (data page)
Chemical composition of living beings
Chemical Galaxy (a new periodic table)
Chemical makeup of the human body
Cosmochemical Periodic Table of the Elements in the Solar System
Natural abundance (isotopic abundance)
Periodic table