The most basic meaning is “a practical application of knowledge” (often “…scientific knowledge…”), but that definition can be used to mean pretty much anything that anybody wants it to mean. I find it more useful to define the concept as “an abstract use of knowledge outside the context of any specific application”. This definition admits to combinations of “knowledge” brought together into a single “use”, without limiting the “technology” to any concrete1 application.
A rigorous means of identifying “technologies” is through the classification of patents. The US patent system originated from the notion that patents are physical documents kept in a drawer. When the drawer filled, they simply started a new one. More recently, the USPTO has clarified and streamline the system, as defined in the Manual of Classification (http://www.uspto.gov/patents/resources/classification/index.jsp). An alternative framework can be found in the World Intellectual Property Organization’s classification scheme. See http://www.wipo.int/ipcpub/#lang=en&refresh=page .
The issue with both classification schemes, from an Engineer’s point of view, is that they are taxonomies (each item falling into exactly one category), whereas our concept of “technologies” are better organized as ontologies. Unfortunately, ontological concepts are difficult to show on the simple tree-type diagrams of which we are so fond.
Footnotes- It will be recalled that one definition of “concrete” is as the opposite of abstract.[↩]