Biennial:
Occurring every two years. The Legislature meets biennially. NOTE: Biannual means "occurring twice a year."
www.leg.state.or.us/glossary.html
Plant that completes its life cycle in two years and generally flowers in the second year
naturesnaturals.co.uk/Botanicals+Glossary
tercentennial
"Fortnight" is a term that means a period of two weeks. So, "every fortnight" means something that occurs or happens once every two weeks.
The term for something that happens by itself is "spontaneous."
The term for something that occurs every ten years is "decennial." This term is derived from the Latin word "decennium," which means a period of ten years. Decennial events can include census counts, anniversaries, and various planning cycles.
"Event" is a term that best describes the phrase "something that happens" as it denotes a happening or occurrence at a specific point in time.
bidecadal
pressure
Phenomenon
The amount of times something happens within a certain period of time. In Herz, and while talking about sound, this would be the amount of waves every second.
Leap year
X chromosome inactivation happens in female mammals.
it is a a centry