Technology tree

A technology tree is what grows out of the ground if you bury a pc in your garden.

Ok....seriously, a technology tree (as used in the Civilization computer games) is as follows:

"As time advances, new technologies are developed; these technologies are the primary way the game changes and grows. Players choose from, at the beginning, advances such as Pottery, the Wheel, and the Alphabet to, at the close of the game, Nuclear weapons and aspects of Space exploration. Players gain a large advantage if their civilization is the first to learn the secrets of flight, for example. Each advance gives access to new units or improvements or derivitive technologies: for example, the Chariot unit becomes available after the Wheel development, and the Granary building becomes available for building after the Pottery development. The whole system of advancements from beginning to end is called the Technology tree, or simply the Tech tree, a concept adopted in many other strategy games."