The concepts of calibration have been detailed by the International Union of Pure and Applied Chemistry (IUPAC) in two documents:

• K. Danzer and L.A. Currie
  Guidelines for calibration in analytical chemistry
  Part 1. Fundamentals and single component calibration
  Pure & Applied Chemistry, 70 (1998) 993-1014
  Download (=1,091 kB: © IUPAC 1998)
  
• K. Danzer, M. Otto and L.A. Currie
  Guidelines for calibration in analytical chemistry
  Part 2. Multispecies calibration
  Pure & Applied Chemistry, 76 (2004) 1215-1225
  Download (=250 kB: © IUPAC 2004)
  

A Part 3, which further elaborates on Section 5. Error diagnosis and validation in Part 2, has recently been published:

• A. Olivieri, N.M. Faber, J. Ferré, R. Boqué, J.H. Kalivas and H. Mark
  Guidelines for calibration in analytical chemistry
  Part 3. Uncertainty estimation and figures of merit for multivariate calibration
  Pure & Applied Chemistry, 78 (2006) 633-661
  Download (=645 kB: © IUPAC 2006)
  

Calibration is defined in Part 1 as follows:

"Calibration in Analytical Chemistry is the operation that determines the functional relationship between measured values (signal intensities y at certain signal positions z_j) and analytical quantities characterizing types of analytes q_i and their amount (content, concentration) x. Calibration includes the selection of the model (its functional form), the estimation of the model parameters as well as the errors, and their validation."

The simplest model is the univariate straight-line fit:

The two bands that accompany the fitted line yield prediction intervals. Part 3 extensively reviews the generalization of univariate methodology to various multivariate and multiway models.