A noosphere was the limit set by a species' technology, biology, and psychology on the information it could find and understand. If a thing or idea existed beyond a species' noosphere, then that thing or idea was either undetectable or incomprehensible to that species.
Pierre Teilhard de Chardin defined a noosphere as the living interaction of thought and culture, complementing a world's geosphere and biosphere. Mictlan notably had a noosphere with neither a biosphere nor a geosphere to support it.
Since it was specifically engineered to be an extension of the Great Houses' will, the Spiral Politic was the Homeworld's noosphere. However, for unknown reasons, the Houses had difficulty penetrating or even just observing certain areas of the Spiral Politic, particularly near the frontier in time. (PROSE: The Book of the War) The Fifth Doctor knew that he wasn't supposed to visit Frontios because it was at the edge of the Gallifreyan noosphere. (PROSE: Frontios) The Book of the War speculated that the Houses' modifications on their own bodies during the War in Heaven were attempts to produce a mutation that could see beyond this edge of their noosphere.
The enemy was thought to come from beyond the Houses' noosphere, since it operated on principles that they weren't built to understand. Chatelaine Thessalia predicted that their first attack would be on the Houses' noosphere; the casts and, later, the babels were designed to protect it.
Certain dead-time states were considered outside of all possible noospheres.