1) a sign, wonder, marvel
2) in a concrete sense, a monster
These two definitions are not unrelated. There are other words in Greek that denote signs, wonders, and marvels. The word teras, being related to the English word terror, indicates that the sign, wonder or marvel brings with it a sense of fear.
Teras is not used in the New Testament, but is found in Greek Translations of the Old Testament. It is used to denote the revelatory aspect of certain events — the event somehow reveals that God makes concrete decisions in the present, that He is in control, and these affect not only the present, but the future. In other words, they are frightening events that remind us that God is the master and creator of the universe, not us. I find this fascinating because of the ramifications it has on role-playing.
In my own experience, the most terrifying opponents in RPGs are humans. The reason for this two-fold. Firstly, having humans as the primary bad guys engenders fear and paranoia because they are not easily identifiable. Whereas an orc is easy to spot, an evil human can be anywhere and be anyone. The second, and more important for this discussion, is that they serve as mirrors — they reflect back at us what is worst in us.
In Orthodox Christian theology, this revelation of sin is understood to be a blessing. It allows us to take control of what is sinful in us, and repent — turn back towards God. This is an unending process that continues until we die and has been compared to purifying gold with fire.
For the purposes of D&D, monsters can be understood as the concrete consequences of sin. God, being creator of everything, including monsters, allows them to exist in order for us to come face-to-face with our own sins — to confront our own monsters and demons, as it were. This brings to life one of my favorite passages from the OT — Genesis 4:6-7 (NJB):
The Lord asked Cain, 'Why are you angry and downcast? If you are doing right, surely you ought to hold your head high! But if you are not doing right, Sin is crouching at the door hungry to get you. You can still master him.'
The word for "sin" in the Hebrew denotes a demon or a monster waiting to devour. This is a marvelous image of our life-long struggle with sin.
Also related to this image is the monastic tradition of Orthodox Christianity. In the Hebrew mind, the desert or wilderness was where demons lived. Thus, monastics would wonder into the deserts and wilderness in order to take on the demons in their own territory — to be that expeditionary force to tame the wilds for the rest of us.
This image, of course, brings to mind the traditional dungeon crawl and hex crawl of old-school D&D. It also reinforces the idea that PCs are that part of civilization whose calling is to go out into the wilderness to confront the demons and monsters in their own territory. In doing so, we are confronted by our own sin and are afforded an opportunity to turn back towards God.
Let me give you a concrete example. One of my all-time favorite pulp authors is H.P. Lovecraft and my favorite monsters in D&D are those that pay homage to Lovecraft's dark vision. For me, these grotesque, hungry, consuming, terrifying creatures and their call represent what awaits creation without God. At the heart of Lovecraft is this sense of inevitable decay, madness and destruction from beyond. At the heart of Orthodox theology is the belief that God created everything from nothing. Without God, all of creation will return to nothing. Lovecraft's call of Cthulu is a personification of this reality. Thus, in terms of D&D, an adventure where PCs enter into a dungeon controlled by Cthulu-inspired monsters is a concrete expression of our own struggle against the nothingness that awaits us if we do not have God to sustain us into eternity.
In other words, these are teras — they are frightening events that reveal to us God and that without Him, we are doomed to the creeping nothing embodied by Lovecraft's horrific visions.