Actually I don't know the difference. I have tried to now the difference between the two but I couldn't that is why I came to this website. Please I need to know because it is part of my agricultural science assignment.
Yes, plant terminology can be confusing. I hope these answer your questions.
A biennial plant is one that takes two years to complete it's life cycle (examples are given in the link below). "Biannual" is a general term not often used with plants, but it can be used to refer to a plant that does something two times (twice) each year (such as produce fruit or flower). Note: This word causes much of the confusion because it can have the same meaning as "biennial" for general English usages and purposes.
A perennial plant is one that lives for more than two years.
An annual plant is one that lives for only one year and must be grown from seed each year.
For the arguments sake, I added the Merriam-Webster dictionary "biannual" definition link below (read about the dual meaning) under "Did you know?" at the link.