What does period mean in the periodic table?

Hint: To answer the given question, we should know about the definition of periodic table. We should know about the arrangement of elements according to their atomic number in the periodic table. The periodic table is presented in a tabular form.

Complete answer:
First, let’s understand the periodic table. The periodic table is the arrangement of all the elements which are known to us in a tabular form according to their atomic number and their recurring chemical properties. It is represented in a tabular form as it consists of rows and columns. Each element present in the periodic table has a principal quantum number (n) for the outermost shell and the valence shell of the element. The principal quantum number represents the period in the modern periodic table.
The period represents the rows in the periodic table whereas the groups represent the column in the periodic table.
All the elements in the row will have the same number of electron shells. Every next element in the row will have an extra proton. The elements arranged in the periods in the periodic table with increasing atomic number left to right. The periodic number can be seen as the row number of the periodic table.
Hence, periods are the horizontal rows (across the periodic table) whereas groups are the vertical columns (down the periodic table).

Note:
At present, there are total \[118\] elements in the periodic table. There are a total of seven periods and eighteen rows in the modern periodic table. Remember that atomic number when we down the group and move across the periods.