When:
Secular became the top lookup on Merriam-Webster on September 16, 2010.
Why:
Pope Benedict XVI's visit to Great Britain, which began on September 16, was often framed as a challenge to "aggressive secularism."
As one paper reported,
"The Pope strongly criticised the 'marginalisation' of Christianity in modern Britain, claiming that churchgoers were forced to act against their conscience in the name of secular equality." - The Telegraph, September 17, 2010
Secular means "of or relating to the physical world and not the spiritual world."
In recent years, as the friction has increased between those who hold religious beliefs and those who do not, secular has seen more use than in decades past.
It comes from a Latin word, saeculum, meaning "the present world."
