Hemisphere
In geometry it is an exact half of a sphere.
It also refers to half of the Earth, such as the "Northern Hemisphere" (that part of the Earth north of the equator), or "Western Hemisphere" (the half of the Earth west of a line running from the North Pole through England to the South Pole, includes the Americas)
- octagon
- octahedron
- octal
- odd number
- odds
- odometer
- one dimensional
- opaque
- open curve
- open interval
- open sentence
- operation
- operator
- opportunity cost
- opposite numbers
- opposite side
- or
- order of magnitude
- order of operations
- ordered pair
- ordering
- ordinal number
- ordinary differential equation
- ordinate
- origin