A somewhat formal or written term for a person who visits a place, such as a home, office, or event. In casual speech, お客さん or 来客 are more common.
We have 10 visitors today.
The parking lot for visitors is this way.
お客さん is the standard polite and casual term for a guest or customer. 来客者 is more formal and impersonal.
Compound of 来 (rai, 'come'), 客 (kyaku, 'guest'), and 者 (sha, 'person'). The exact historical derivation is uncertain, but the components clearly indicate 'person who comes as a guest'.