A basilica is a large, important church. The word can also be used for an Ancient Roman building that was used for law and meetings. The word "basilica" is Latin which was taken from the Greek "Basiliké Stoà".
Nowadays the word is used in three ways:
A Roman Catholic church that has been given the right to use that name, by the Pope. Only some large important churches have this right.
An Ancient Roman basilica
People who write about architecture often use the word "basilica" to mean a building that is shaped like an Ancient Roman basilica.