Definitions

Monolingual: A person knowing only one language (40% of world population)
Bilingual: A person using or able to use two languages especially with equal fluency (43% of world population)
Trilingual: A person speaking three languages fluently (13% of world population)
Multilingual: A person who speaks more than two languages, but used often for four languages or more (3% of world population speak more than 4 languages)
Polyglot: Someone with a high degree of proficiency in several languages (less than 1% of world population speaks 5 languages fluently)