Sentence:  Economics is a messy discipline: too fluid to be a science, too rigorous to be an art. Perhaps it is fitting that economists' most-<WSD>used</WSD> metric, gross domestic product (GDP), is a tangle too. GDP measures the total value of output in an economic territory. Its apparent simplicity explains why it is scrutinised down to tenths of a percentage point every month. 
Meaning of the word marked with <WSD> tags:  That is or has or have been used.