In cognitive psychology, working memory is a theoretical framework that refers to structures and processes used for temporarily storing and manipulating information. As such, working memory might also just as well be referred to as working attention. The term was first used in the 1960s in the context of theories that likened the mind to a computer. A second source of the concept of working memory is the distinction between short-term memory and long-term memory. Most theorists today use the concept of working memory to replace or include the older concept of short-term memory, thereby marking a stronger emphasis on the notion of manipulation of information instead of passive maintenance.
Working memory is generally considered to have limited capacity. The earliest quantification of the capacity limit associated with short-term memory was the "magical number seven" introduced by Miller (1956). He noticed that the memory span of young adults was around seven elements, called chunks, regardless whether the elements were digits, letters, words, or other units. Later research revealed that span does depend on the category of chunks used (e.g., span is around seven for digits, around six for letters, and around 5 for words), and even on features of the chunks within a category. For instance, span is lower for long than for short words. In general, memory span for verbal contents (digits, letters, words, etc.) strongly depends on the time it takes to speak the contents aloud, and on the lexical status of the contents (i.e., whether the contents are words known to the person or not) (Hulme et al., 1995). Several other factors also affect a person's measured span, and therefore it is difficult to pin down the capacity of short-term or working memory to a number of chunks. Nonetheless, Cowan (2001) has proposed that working memory has a capacity of about four chunks in young adults (and less in children and old adults).