Ground State
Definition
An atom's lowest-energy, most stable arrangement of electrons: its normal resting condition.
Technical definition
The ground state is the lowest possible energy state of an atom or molecule, with electrons occupying the lowest available orbitals according to the Aufbau principle. Electron configurations are written for the ground state unless stated otherwise.
Example
A hydrogen atom's ground state has its single electron in the 1s orbital. Any electron in n=2 or higher means the atom is excited.
Common confusion
Ground state refers to energy, not temperature or physical location. An atom at room temperature is still in its electronic ground state unless it has absorbed energy.