The body nimbus perceptible to a sime caused by the presence of selyn, mathematically akin to the electric field surrounding an electron. (See also: Nager) "Gradient" refers to the gradual fall-off of intensity with distance from the subject.
(Source: Index card file. Sime Surgeon, http://www.simegen.com/sgfandom/rimonslibrary/surgeon/SURGEON1.html) Describing Ilyana's field --- "a throbbing nimbus of energy."
A sort of pulling force --- like gravity which falls off with distance. A difference between fields, the lesser tending to flow into the former.
(Source: Index card file. Operation High Time, http://www.simegen.com/sgfandom/rimonslibrary/oht.html) Possessed by both simes and Gens. Gradient can be a field on its own, doesn't need 2 or more people. --- "with no field gradient other than my own to distract me."