Exotek (exotic technologies) is used to describe technologies not generally available in the Terrasphere. Exotek is highly sought after, and many fortunes have been made, and even more lives lost hunting for it.
After the fall, few sites in the Terrasphere retained the ability to construct or even maintain the more sophisticated technologies, so lost or hidden caches of such lost technology were highly valuable. Perhaps the most important example was the recovering of an intact member of the 'Seven Sisters' in the 32C which enabled interstellar travel to recommence.
Some forms of exotek are illegal technologies that are banned in the Terrasphere, such as high level AIs, sentient nanites, and any derivitives of virus-z. 
Some 'exotek' has turned out to be of Alien rather than human origin. Some are very old, around 1.5 million years old, and belonging to the so called Kalron Culture, believed to be the race that built the Pyramid on Rubina II. Some examples of alien exotek are newer, and appear contemporous with the Terrasphere. Their origin, as well as their function are unknown.