Period
A period is a horizontal row of the periodic table; all elements in a period have the same number of electron energy levels.
Definition
A period is a row going across the periodic table from left to right, and there are $7$ of them. The period number tells how many occupied electron energy levels an atom of that element has, and moving left to right each element has one more proton and one more electron than the last, so properties change gradually from reactive metals to reactive nonmetals to an unreactive noble gas. Each period corresponds to the filling of a principal quantum shell $n$, and its length equals the number of orbitals available: $2$ elements for $n = 1$ ($1s$), $8$ for periods $2$ and $3$ ($s$ and $p$), $18$ for periods $4$ and $5$ (adding $d$), and $32$ for periods $6$ and $7$ (adding $f$). Across a period, effective nuclear charge increases while shielding from inner shells stays roughly constant, so atomic radius decreases and ionization energy and electronegativity generally increase.
Example
The first period has only $2$ elements, hydrogen and helium, and the second runs from lithium to neon, $8$ elements. Period $3$ runs from sodium ($Na$, $11$ protons) to argon ($Ar$, $18$ protons); all eight have three energy levels, sodium has $1$ valence electron, chlorine has $7$, and argon has a full $8$. Across period $2$, first ionization energy climbs from lithium ($520\text{ kJ/mol}$) to neon ($2081\text{ kJ/mol}$), with small dips at boron and oxygen caused by subshell and electron-pairing effects, while atomic radius falls from $152\text{ pm}$ (Li) to about $69\text{ pm}$ (Ne).
Key Insight
As you move across a period the elements change a lot, from a shiny metal on the left to a colorless gas on the right. Period number equals the number of electron shells, so elements in period $4$ are bigger than the elements right above them in period $3$, and atoms get larger going down the table. Period $7$ was completed in 2016 with the naming of nihonium, moscovium, tennessine, and oganesson, and any element $119$ or beyond would begin period $8$ and, in theory, start filling a $g$ subshell.