An artist's depiction of the noosphere
The Noosphere, according to the thought of Vladimir Vernadsky and Teilhard de Chardin, denotes the "sphere of human thought". The word is derived from the Greek νοῦς (nous "mind") + σφαῖρα (sphaira "sphere"), in lexical analogy to "atmosphere" or "biosphere". In relation to the S.T.A.L.K.E.R. games, scientific experiments done on the noosphere formed the Zone.
Description
In the original theory of Vernadsky, the noosphere is the third in a succession of phases of development of the Earth, after the geosphere (inanimate matter) and the biosphere (biological life). Just as the emergence of life fundamentally transformed the geosphere, the emergence of human cognition fundamentally transforms the biosphere.
For Teilhard, the noosphere emerges through and is constituted by the interaction of human minds. The noosphere has grown in step with the organization of the human mass in relation to itself as it populates the earth. As mankind organizes itself in more complex social networks, the higher the noosphere will grow in awareness.
The Noosphere and the Zone
In S.T.A.L.K.E.R. game series, the theory of noosphere is altered slightly. Rather than an (more or less) abstract social/environmental concept, the noosphere is very much real, a tangible if invisible field surrounding the Earth linked by, affected by, or affecting both human minds and thoughts.
After the 1986 incident at the Chernobyl Nuclear Power Plant, a series of secret laboratories were created within the 30 kilometer exclusion zone around the Chernobyl NPP. The Noosphere was one of the main research areas, and the scientists concluded that it is indeed real and could be tampered with - removing negative emotions such as anger, cruelty, greed, etc. entirely from the planet. However, a single human could never be able to affect the noosphere in any noticeable way; thus it led to creation of the C-Consciousness.
In 2006, when C-Consciousness attempted to alter the noosphere, the experiment backfired horribly. The result was the Zone. Since then, the C-Consciousness has been fighting against the Zone, with little success. Mostly by attempts to contain it.
In one of the endings of S.T.A.L.K.E.R.: Shadow of Chernobyl, it is suggested that destroying the C-Consciousness also resulted in destruction of the Zone. However, in Call of Pripyat, it's revealed that the Zone is still very much active, and the damage to noosphere is likely permanent.