In computer science terms efficiency is the ratio of useful work to resources (processor and storage) expended. In other words, the ratio of the output to the input of a given system. If for example an algorithm is said to be efficient, it does a good job of economising on the computer resources to achieve its objective.
In HCI or Usability terms, efficiency is measured as the resources expended by the user in relation to the accuracy and completeness of goals achieved (ISO standard 9241). High efficiency is achieved when the user reaches his/her goals while expending as few resources as possible according to the mentioned ISO standard. This definition is however very business-oriented and only partly applies to software such as games or likewise.
