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In physics and chemistry, 'monatomic' is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a 'monatomic gas' is one in which atoms are not bound to each other.
Under Standard Temperature and Pressure (STP), all of the noble gases are monatomic. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently large temperatures.
The only mode of motion of a monatomic gas is translation (electronic excitation is not important at room temperature). Thus in an adiabatic process, monatomic gases have an idealised $gamma$-factor (C_p/C_v) of $5/3$, as opposed to $7/5$ for ideal diatomic gases where rotation (but not vibration at room temperature) also contributes, and:
:The heat capacity for monatomic gases at constant pressure (C_P) is 2.5 R = 20.8 J.mol^-1.K^-1
:The heat capacity for monatomic gases at constant volume (C_V) is 1.5 R = 12.5 J.mol^-1.K^-1
where R is the gas constant.