Affinity is a quasi-telepathic link that was developed by the Edenists in the late 21st Century. They use it to communicate mentally with each other, with their habitats and voidhawks. By the 27th Century, it has been further developed to allow direct control of non-sentient computers (such as a bitek array) and animal life forms (such as spiders for remote sensing).
Originally, affinity was achieved by neuron symbionts. That is, an implant in the brains of the "sender" and "receiver", allowing them to contact each other. Eventually, affinity was encoded in human DNA and embryos engineered to carry this gene. This was one of the reasons which caused the Edenist community to be ex-communicated from the Church (the other being the transfer of Edenist memories and personalities to the neural strata of the original habitat Eden).
Affinity has two modes of communication. One is a secure person-to-person mode for privacy, the other is communial, allowing all Edenists in a given location to pick up on the conversation. Taking the communial mode further, Edenists have a "Consensus", kind of mega-meeting of minds, to discuss matters of grave concern (i.e possesion or the Laton crisis).
One of the misconceptions of affinity that Adamists often have is that affinity is "a voice inside your head". It is so much more, as a full range of emotions, visual images, smells, and sounds can be transmitted.
Affinity works using an odd property of physics known as quantum entanglement. When two particles are entangled in such a way, the two are linked at a fundamental level, no matter how far apart they are. This allows affinity to broadcast data etc without any apparent means of transmission (i.e. radio), which renders the link to be undetectable of non-affinity technology. It also allows faster-than-light communication. In reality, the distance that affinity has is limited by the power of the sender and the interference present such as large quantities of matter.
Affinity is not used exclusively by Edenists. Owners of blackhawk starships (who are usually Adamist) will have neuron symbionts implanted which allows affinity communication with their ship. Unlike Edenist affinity, the owners link is "tuned in" to ship only. Blackhawks do have general affinity though.
Another notable use of affinity (or a perversion of it, as seen from the Edenist standpoint) is that of the habitat Valisk. The personality of habitat, Rubra controls his descendants via a modified affinity in the hope that one day these decendants will make Valisk great again.