Religio

The Latin term religiō, the origin of the modern lexeme "religion" (via Old French/Middle Latin), is of ultimately obscure etymology. It is recorded beginning in the 1st century BC, i.e. in Classical Latin at the end of the Roman Republic, notably by Cicero, in the sense of "scrupulous or strict observance of the traditional "cultus"". In classic antiquity, it meant conscientiousness, sense of right, moral obligation, or duty towards anything and was used mostly in secular or mundane contexts.
Etymology.
The classical etymology of the word, traced to Cicero himself, derives it from "relegere": re (again) + lego (read) where lego is in the sense of "go over", "choose", or "consider carefully". Modern scholars such as Tom Harpur and Joseph Campbell have argued that "religio" is derived from "religare": re (again) + "ligare" (bind or connect), which was made prominent by Augustine of Hippo, following the interpretation of Lactantius in "Divinae institutiones", IV, 28.
Newer research shows that in the ancient and medieval world, the etymological Latin root "religio" was understood as an individual virtue of worship in mundane contexts; never as doctrine, practice, or actual source of knowledge. In general, "religio" referred to broad social obligations towards anything including family, neighbors, rulers, and even towards God. "Religio" was most often used by the ancient Romans not in the context of a relation towards gods, but as a range of general emotions such as hesitation, caution, anxiety, fear; feelings of being bound, restricted, inhibited; which arose from heightened attention in any mundane context. The term was also closely related to other terms like "scrupulus" which meant "very precisely" and some Roman authors related the term "superstitio", which meant too much fear or anxiety or shame, to "religio" at times. When "religio" came into English around the 1200s as religion, it took the meaning of "life bound by monastic vows" or monastic orders.
Examples of usage.
Cicero connected "" read, i.e. "re" (again) with "lego" in the sense of choose, go over again or consider carefully. The definition of "religio" by Cicero is "cultum deorum", "the proper performance of rites in veneration of the gods."
Julius Caesar used "religio" to mean "obligation of an oath" when discussing captured soldiers making an oath to their captors
The Roman naturalist Pliny the Elder, used the term "religio" to describe elephants' supposed veneration of the sun and the moon.
St. Augustine, following the interpretation given by Lactantius in "Divinae institutiones", IV, 28 derived "religio" from "re" (again) and "" bind, connect, probably from a prefix. "
The medieval usage alternates with "order" in designating bonded communities like those of monastic orders: "we hear of the 'religion' of the Golden Fleece, of a knight 'of the religion of Avys'".
Significance in Roman religion.
Within the system of what we would now call "Roman religion (in the modern sense of the word), the term "religio" originally meant an obligation to the gods, something expected by them from human beings or a matter of particular care or concern as related to the gods, 
"reverence for God or the gods, careful pondering of divine things, piety".
In this sense, "religio" might be translated better as "religious scruple" than with the English word "religion". One definition of "religio" offered by Cicero is "cultus deorum", "the proper performance of rites in veneration of the gods."
"Religio" among the Romans was not based on "faith", but on knowledge, including and especially correct practice. "Religio" (plural "religiones") was the pious practice of Rome's traditional cults, and was a cornerstone of the "mos maiorum", the traditional social norms that regulated public, private, and military life. To the Romans, their success was self-evidently due to their practice of proper, respectful "religio", which gave the gods what was owed them and which was rewarded with social harmony, peace and prosperity.
Religious law maintained the proprieties of divine honours, sacrifice, and ritual. Impure sacrifice and incorrect ritual were "vitia" (faults, hence "vice," the English derivative); excessive devotion, fearful grovelling to deities, and the improper use or seeking of divine knowledge were "superstitio"; neglecting the "religiones" owed to the traditional gods was atheism, a charge leveled during the Empire at Jews, Christians, and Epicureans. Any of these moral deviations could cause divine anger ("ira deorum") and, therefore, harm the State. See Religion in ancient Rome.
"Religiosus" was something pertaining to the gods or marked out by them as theirs, as distinct from "sacer", which was something or someone given to them by humans. Hence, a graveyard was not primarily defined as "sacer" but a "locus religiosus", because those who lay within its boundaries were considered belonging to the di Manes. Places struck by lightning were taboo because they had been marked as "religiosus" by Jupiter himself.