|An 1845 encyclopedia definition of “secular”, meaning something temporal, as opposed to ecclesiastical. |
In 1851, George Holyoake introduced, and or somehow popularized, the term “secularism” (or secularists) as an upgrade to “freethought” (or free thinkers), but less derisive or more preferable to some than “atheism”; meaning indifference to or rejection or exclusion of religious considerations. 
In 1856, Henry Buckle, in his History of Civilization, discussed how Richelieu, from 1625 to 1643, established the world’s first “secular scheme” of government, in opposition to the "ecclesiastical scheme".
In 2014, American sociologist and secularism scholar Phil Zuckerman defined a secular, simply as non-religious; and to elaborate, in respect to what "religious" means, he elaborates that a secular person is someone who does not practice the "three Bs", namely: belief (in supernatural), behavior (rituals, rites, or behaviors practices, actualized according to those beliefs), and belonging (belong or identify with a group that is religious, anchored in those beliefs and behaviors). 
Some have characterized Thucydides (Ѻ) as a secular atheist.
The term "movement atheism", in atheism terminology, supposedly, those atheists who are “on board” with the atheism movement and ignore the various kinds of atheists who are uninterested in it or hostile to it; of which there are four kinds: secularist atheists, identity atheists, evangelical atheists, and constructive atheists. (Ѻ)
1. (a) Holyoake, George. (1871). The Principles of Secularism Illustrated. Book Store.
(b) Holyoake, George. (1896). The Origin and Nature of Secularism: Showing that where Freethought Commonly Ends Secularism Begins. Watts.
(c) Smith, George. (1991). Atheism, Ayn Rand, and Other Heresies (pg. 172). Prometheus Books.
(d) Secularism – Merriam-Webster Collegiate Dictionary, 2000.
2. (a) Zuckerman, Phil. (2015). “Op-Ed: How Secular Family Values Stack Up” (Ѻ), Jan 14.
(b) Moore, Tracy. (2015). “Godless Parents are doing a Better Job” (Ѻ), Jezebel.com, Feb 2.
3. Maunder, Samuel. (1845). The Scientific and Literary Treasure: a New and Popular Encyclopedia (pg. 681). Publisher.
● Zuckerman, Phil. (2014). Living the Secular Life: New Answers to Old Questions. Penguin.
● Secularism – Wikipedia.