Secularization, as understood by sociologists of religion, is a varied term with multiple definitions and levels of meaning. It should also be noted that in addition to multiple definitions, secularization is both a theory and a process. By theory, it is meant that some scholars (e.g. Marx, Freud, Weber, Durkheim) believed that as society modernized it would also see a decline in levels of religiosity. This understanding of classical secularization theory is currently being refined and modified (see discussion below). The 'process' component of secularization would refer to how the theory is actualized. It is in this sense that secularization has multiple definitions. The most common meaning is in reference to the decline of levels of religiosity in society, but this is a broad and diffuse meaning that should be clarified by referring to one of the more specific meanings outlined below.

Sommerville (1998) outlined six (6) uses of the term secularization in the scientific literature. The first five are more along the lines of 'definitions' while the sixth application of the term is more of a 'clarification of use' issue:
(1) When discussing social structures, secularization can refer to differentiation. Differentiation (or specialization) is a reference to the increasing division of labor and occupational specialization in society. While some might consider this a foray into social 'progress', few would argue that modern societies are less differentiated than more primitive/traditional societies (following the work of Gerhard Emmanuel Lenski ).
(2) When discussing institutions, secularization can refer to the transformation of an institution that had once been considered religious in character into something not thought of as religious. A good example of this type of secularization (and differentiation, for that matter) is the transition of Harvard University from a predominantly religious institution into a secular institution (with a divinity school now housing the religious element illustrating differentiation).
(3) When discussing activities, secularization refers to the transfer of activities from institutions of a religious nature to others without that character. While the trend toward government assistance in social welfare seems to be reversing in recent years, for much of the 20th century activities that had been in the religious domain - e.g. soup kitchens - were slowly moving into the secular (or a-religious) realm, often that of government.
(4) When discussing mentalities, secularization can refer to the transition from ultimate concerns to proximate concerns. This is the most common understanding and usage of the term at the individual level and refers specifically to personal religious decline or movement toward a secular lifestyle.
(5) When discussing populations, secularization can refer to a societal decline in levels of religiosity (as opposed to the individual-level secularization of definition four). It should be noted that this understanding of secularization is distinct from definition one (1) in that it refers specifically to religious decline rather than societal differentiation. A clear example of this definition of secularization would be the declining religious affiliations in much of modern Europe.
(6) When discussing religion generally, secularization can only be used unambiguously when referring to religion in a generic sense. For example, to argue that Christianity is 'secularizing' is not clear unless one specifies exactly which elements of which version of Christianity are being discussed. What's more, depending on the venue of the discussion, these elements of Christianity may not be recognized by other 'Christian' groups as elements of their version of Christianity. Thus, if you are interested in discussing religious decline within a specific denomination or religion, you need to specify which elements of that specific group you believe are declining, as Christianity is too variably defined to allow for generalizations for a specific denomination.

Current Issues in the Study of Secularization
At present, secularization (as understood in definition five above) is being debated in the sociology of religion. Some scholars (e.g. Rodney Stark) have argued that levels of religiosity are not declining (though their argument tends to be limited to the U.S., an admitted anomaly in the developed world). As there appears to be some merit to this position, other scholars (e.g. Mark Chaves, N.J. Demerath) have countered by introducing the idea of neo-secularization, which broadens the definition of individual level religious decline by arguing that secularization can also refer to the decline of religious authority. In other words, rather than using a-religious apostates as the solitary measure of a population's secularity, neo-secularization argues that individuals are increasingly looking outside of religion for authoritative positions on different topics. Neo-secularizationists would argue that religion is no longer the authority on issues like whether to use birth control and would therefore argue that while religious affiliation may not be declining in the U.S. (a debate still taking place), religion's authority is declining and secularization is taking place.

References: Chaves, M. 1994. "Secularization As Declining Religious Authority." Social Forces 72(3):749-74.

Sommerville, C. J. 1998. "Secular Society Religious Population: Our Tacit Rules for Using the Term 'Secularization'." Journal for the Scientific Study of Religion 37 (2):249-53.

Stark, Rodney, Laurence R. Iannaccone, Monica Turci, and Marco Zecchi. 2002. "How Much Has Europe Been Secularized?" Inchiesta 32(136):99-112.

Note: This page was redirecting to Secularism, but secularism is not the same as secularization.