S.M.A.R.T. is a mnemonic acronym, giving criteria to guide in the setting of objectives, for example in project management, employee-performance management and personal development. The letters S and M generally mean specific and measurable. Possibly the most common version has the remaining letters referring to achievable (or attainable), relevant, and time-bound. However, the term's inventor had a slightly different version and the letters have meant different things to different authors, as described below. Additional letters have been added by some authors.