Despite its importance, there isn't always a great deal of agreement on just what secularism really is. Part of the problem lies in the fact that the concept of "secular" can be used in a couple of ways which, while closely related, are nevertheless different enough to make it difficult to know for sure what people might mean. The word secular means "of this world" in Latin and is the opposite of religious. As a doctrine, secularism is usually used to describe any philosophy which forms its ethics without reference to religious dogmas and which promotes the development of human art and science.
The term secularism was created in 1846 by George Jacob Holyoake in order to describe "a form of opinion which concerns itself only with questions, the issues of which can be tested by the experience of this life" (English Secularism, 60). Holyoake was a leader of the English secularist and freethought movement who became famous to the wider public for his conviction under, and larger fight against, English blasphemy laws. His struggle made him a hero for English radicals of all types, even those who were not members of freethought organizations.
Holyoake was also a social reformer who believed that the government should work for the benefit of the working classes and poor based upon their needs in the here and now rather than any needs they might have for a future life or for their souls. As we can see from the quote above, his early usage of the term "secularism" did not explicitly portray the concept in opposition to religion; rather, it only refers in passing to the idea of focusing upon this life rather than speculation about any other life. That certainly excludes many religious belief systems, most importantly the Christian religion of Holyoake's day, but it doesn't necessarily exclude all possible religious beliefs.
Later, Holyoake explained his term more explicitly:
Secularism is that which seeks the development of the physical, moral, and intellectual nature of man to the highest possible point, as the immediate duty of life which inculcates the practical sufficiency of natural morality apart from Atheism, Theism or the Bible which selects as its methods of procedure the promotion of human improvement by material means, and proposes these positive agreements as the common bond of union, to all who would regulate life by reason and ennoble it by service" (Principles of Secularism, 17).
Once again we see a focus upon the material and upon this world rather than the immaterial, the spiritual, or any other world but we also don't see any specific statement that secularism involves the absence of religion. The concept of secularism was originally developed as a non-religious philosophy focused upon the needs and concerns of humanity in this life, not the possible needs and concerns associated with any possible afterlife. Secularism was also designed as a materialistic philosophy, both in terms of the means by which human life was to be improved and in its understanding of the nature of the universe.
Today, such a philosophy tends to be labeled humanism or secular humanism while the concept of secularism, at least in the social sciences, is much more restricted. The first and perhaps most common understanding of "secular" today stands in opposition to "religious." According to this usage, something is secular when it can be categorized with the worldly, civil, non-religious sphere of human life. A secondary understanding of "secular" is contrasted with anything that is regarded as holy, sacred, and inviolable. According to this usage something is secular when it is not worshipped, when it is not venerated, and when it is open for critique, judgment, and replacement.