The 1960 U.S. census began to statistically define American cities with the label "Standard Metropolitan Statistical Areas (SMSAs). In 1983, the government dropped the term MSA in favor of MSA (Metropolitan Statistical Area). They were defined on the basis of central cities (or twin cities) with a combined population of 50,000 or more, together with surrounding counties (or towns, in New England) that are metropolitan in character (as measured by population density and the percentage of the population living in urban areas) or that are economically and socially l linked to the central city (as measured by commuting patterns).
At the same time, the expansion of large neighboring metropolitan areas led to the addition of the concept of a Consolidated [Metropolitan Statistical Area (CSMA), a large metropolitan complex with at least one million inhabitants and two or more Primary Metropolitan Statistical Areas (PMSAs). A county or group of counties may qualify as a MSA on the basis of either MSA status or local perceptions of relative independence within the larger metropolitan complex. The Bureau of the Census now recognizes, however, that our cities and metropolitan areas have outgrown these definitions, and it is currently engaged in a major revision of its terminology and definitions."
from: Urbanization: An Introduction to Urban Geography by Paul L. Knox 1994 Prentice Hall Inc. p.6