Lookups spiked on May 12, 2015.
A poll on religious affiliation in America found that fewer people identify themselves as "Christian," and, correspondingly, that the number of people who claim to belong to no organized religion has increased. According to the study, conducted by the Pew Research Center: "As the ranks of the religiously unaffiliated continue to grow, they also describe themselves in increasingly secular terms," including "atheists," "agnostics," or "nothing in particular."
Secular means "of or relating to the physical world and not the spiritual world" or "not religious." It comes from the Latin word that evolved from meaning "generation" or "age" to meaning "century" (taken as the extreme limit of a human lifetime). It came to mean "the present world," as opposed to the seclusion of monastic religious life, in order to identify clergy who worked in churches among the people. From there, it was only a step to today's meaning of "not religious."
Secular came to English from Latin through French; in modern French the word siècle means "century."