Definition of sentiment analysis
A linguistic analysis technique where a body of text is examined to characterise the tonality of the document. Though the method pre-dates modern technological tools, the use of sentiment analysis has accelerated in recent years with the development of large-scale computational infrastructure that can analyse large unstructured textual data sets.
Given the growth of user-generated content, sentiment analysis is useful in social media monitoring to automatically characterise the overall feeling or mood of consumers as reflected in social media toward a specific brand or company. Thomson Reuters uses sentiment analysis in a number of its different products to provide traders with information about how companies are faring in news articles. 
By monitoring trends on sites such as Twitter, as well as comments on popular blogs, companies are using software to carry out 'sentiment analysis' around their brands. This, for example, can give managers an early warning of a customer service issue, or even that a competitor has come to market with a more attractive offering.
These tactical insights are valuable to businesses, because they can often provide quicker feedback than conventional data sources, such as customer service records or point of sale systems. By the time a business notices a fall-off in sales from its back office systems, the customer has already moved on.