Think tank is a slang term used to describe a robotic weapons platform that makes use of artificial intelligence to enhance its abilities. Most of the think tanks portrayed in Ghost in the Shell (manga, films, and series), along with other machinery, (e.g. attack helicopters such as the Jigabachi AV) are developed and manufactured by a fictional company named Kenbishi Industries.
Think tank is often used to refer to the different walking tanks throughout the Ghost in the Shell world. Typically they are spider-like in appearance, having four to six walking legs, usually a pair of front-mounted manipulators, and a segmented body. Models include the Tachikoma, shown throughout the Stand Alone Complex Series, the Fuchikoma, found in the original manga, the Uchikoma, seen briefly at the end of SAC: 2nd Gig and in the beginning of Solid State Society, and the Logikoma, which appears in GITS Arise. The various -koma can also be found in video games based on the series.
There are other prominent models as well, such as the small walking tank found at the end of the original Ghost in the Shell film, the main battle tank-sized "HAW206" model featured in episode two of Ghost in the Shell: Stand Alone Complex and the large "Type 3" think tank seen several times during 2nd Gig, recognisable from its large, conventional tank-like turret and four caterpillar-tracked legs.