No one will ever ask you to define a Period.
Frankly, all you need to know is that they are the horizontal rows of the Periodic Table.
You should also know that a Period does not always contain orbitals from only one shell. The fourth period contains 4s and 4p orbitals but also 3d orbitals.
People frequently forget to count the top period (Period 1 = H and He) - be careful!
A level Chemistry does not involve Actinides and Lanthanides but, as in the diagram above, they are included in Periods 6 and 7