Period
Definition
A horizontal row of the periodic table. The period number tells you how many electron shells an atom's electrons occupy.
Technical definition
A period is a horizontal row (numbered 1-7) of the periodic table. The period number equals the number of occupied principal electron shells in the ground state, and properties change gradually across a period as protons and electrons are added.
Example
Sodium is in period 3: its electrons occupy three shells, with the outermost electron in the third shell.
Common confusion
Elements in the same period do NOT behave similarly. Behavior repeats down a group, while across a period properties shift steadily from metallic to nonmetallic.