and others have pointed out, the word 'techno' started life in the mid-1980s to refer to the the Detroit
electronic sounds that gave rise to house
However the word can be used in many ways. The word techno, as it started to be used in the early 1990s, has two distinct endpoints to the range of its meanings.
Firstly, techno in the broad sense, means all music created entirely or mostly by digital electronic equipment. At first this was called rave music, but soon diversified into styles suitable for other occasions than just dancing all night, and many styles suitable for that.
Techno is a genre of the same magnitude as Rock and roll or jazz, with many flavours. Thus we can refer to Big beat, breakbeat and ambient as "Big beat techno", "breakbeat techno" and "ambient techno".
Techno encompases a lot of dancable music, but also ambient and other styles. Machines can make any noise, really, and techno is continually diversifying. This sense of the word techno is very close to that is now often called electronica. (See my rant there on the present and future of electronic music).
Secondly, techno in the narrow sense is a particular style of techno, a minimalist, fast four on the floor dancable beat and groove that emerged in Detroit in the mid 1980s and developed in Europe in the early 1990s.