What are the terms that describe these values for the element oxygen?

Answer: 18 = Mass umber
8 = atomic number
Explanation:
An atom is smallest unit of a matter or substance.It consist of three subatomic particles : electrons, protons, neutrons.
Protons and neutrons are located in the center of an atom where while mass of an atom is concentrated.
Mass number = Number of protons + Number of neutrons
Atomic number = Number of protons = Number of electrons (for a neutral atom)
General representation of an element is given as: [tex]_Z^A\textrm{X}[/tex]
where,
Z represents Atomic number
A represents Mass number
X represents the symbol of an element
Oxygen is the 8th element of the periodic table and is written as [tex]_8^{16}\textrm{O}[/tex], thus 8 = atomic number and 18= Mass number