Period
A horizontal row of the periodic table, corresponding to one principal quantum number of the valence shell.
Unit
Dimensionless
In depth
Moving left to right along a period, effective nuclear charge rises while the shell number stays fixed, so atoms shrink, ionisation energy and electronegativity climb, and character shifts from metallic through metalloid to non-metallic. Period length follows subshell capacity: 2, 8, 8, 18, 18, 32, 32.
Examples in the real world
Period 3 runs from sodium, a soft reactive metal, to argon, an inert gas, with silicon as the metalloid in between.