From Halopedia, the Halo wiki
Casimir Station is a space station that is presumably in orbit of Reach.
On the morning of August 30, 2552, Ensign William Lovell was in contact with Casimir Station, requesting a check on the sensors of the UNSC Pillar of Autumn, which was picking up unusual EM readings bouncing off Turul. Unbeknownst to the crew of the ship, which was being kept ignorant of the Covenant invasion, the readings were actually a result of battles occurring on the other side of the planet.
Casimir Station shares its name with the city of Casimir on the planet Reach. This may indicate the station to be an Erőd-class orbital weapon platform, following the trend of naming OWP stations after cities on the planet below them (ie Quezon Station after Quezon on Reach, or Cairo Station after Cairo on Earth). It is possible that it was named after Hendrik Casimir, one of the scientists who discovered the Casimir effect.