Go concepts

Go to a new player is merely a mechanical thing. But as one advances, one becomes aware of a subtlety beneath stronger play that trancends calculable mechanical thinking.

Conceptual thinking is what allows intelligent beings to deal with very complex issues, with some basic rules of understanding. This exists in go to a degree greater than in other games, as placed stones exert a hard-to-quantify influence of play over the board, and these effects need to be understood for a player to rise to the dan ranks.

Concepts

Sente & gote

Sente (gain of initiative) and its opposite, gote. (loss of initiative) - See Go strategy and tactics

Miai

A pair of points on the board that are equivalent in terms of value with respect to a groups development or survival. Miai can be seen in the fuseki stage on a large scale or in a simple life and death problem, like a straight four-space eye. This shape is alive, because if white plays b, black can answer with a and vice versa.

Aji

The closest English we could use is 'latent potential.' From the Japanese, Aji is the word for taste, and in go refers to the lingering quality that even dead stones will provide possible avenues of subtle play. Though aji may not be used at all, it has a bearing on the course of the game. Good aji is when your groups are strong, and have little or no possibility of being compromised. Bad aji, is where dead stones carry a latent threat of compromising an existing area, should the situation become ripe.

Korigatachi

Frozen shape. To force your opponent into sequences that make weaker, less influential and efficient shapes. Conversely, knowing something of korigitachi should tell you how to avoid it.

Sabaki

Light and flexible shape development.

Thickness

Also called influence. Thickness refers to a kind of influence, where an area is developed beyond the level of the area around it. A large wall, for example, is a common example of thickness, and if that wall has no compromising weaknesses in it, will provide a help to its other stones in the area.

Kikashi

Kikashi is a forcing move in the context of an attack. Unlike sente, though, a move is kikashi, when it yields a high efficiency in play by causing the opponent to regard that move in making a change in its course of action.
Moves can be kikashi, or not, depending on whether they are answered with appropriate sophistication or not.

Yosu-miru

A probe. A yosu-miru move is, in some sense, a sacrifice of a stone, but is designed to yield a very sophisticated kind of information about a developing group and how best to attack it, based on its response. Yosu-miru draws on other concepts of kikashi and aji, and korigatachi in order to understand it fully.