This refers to a plant that normally completes its entire life cycle in one year or less. Cannabis is a highly adaptive annual plant that grows throughout temperate and tropical zones worldwide.
Merriam-Webster Online Dictionary
annual (adjective)1.
covering the period of a year - annual rainfall
2.
occurring or happening every year or once a year - yearly an annual reunion
3.
completing the life cycle in one growing season or single year - annual plants
1.
an event that occurs yearly
2.
a publication appearing yearly
3.
something that lasts one year or season , specifically a plant that completes its life cycle in one growing season
Yearly, Perennial, Seasonal, Once-a-year, Cyclical