When something changes from being closely connected to or controlled by religion to being nonreligious, that's secularization. A religious school that's ending its affiliation with a church is undergoing secularization.
An institution, government, or society that has no connection to religion is secular, and the process of moving toward being secular is secularization. If a country's population has become less religious or spiritual over time, it's evidence of the secularization of the entire society. Many colleges, including Harvard University for example, were religious institutions until they underwent secularization.