Science
Science is the art of trying to describe the world around us as accurate as possible.
This is done in a very systematic fashion, using step by step reasoning from hypothesis to proof using verifyable experiments along the way.
Thus you might hypothesize that it would be neat to turn lead into gold, spend a lifetime on achieving this feat through various experiments, come to the (mistaken) conclusion that it is not possible and give up.
Or, along the way you might stumble on radioactivity, win a nobel prize or two, realise (correctly) that it is possible using atomic reactions to achieve your goal only to find that this is not economically feasible.
In both cases you will probably learn a lot !
Science does away with 'belief' and replaces it with 'peer review' and 'proof'.
A scientist is not swayed by someone imploring them to believe their results, he or she wants to be able to do the experiments all over from scratch to see if they really work. This is called peer-review, and only after another scientist has successfully replicated the experiment will it get published in a more reputable scientific publication.
The purpose behind science is to make sure that we spend our time on fruitful areas of research, cull off 'dead ends' and to keep a record of all that we have discovered.
Until the advent of modern science 'magic' or 'god' was usually quoted as the moving force behind something not (or not well) understood.
So, we had gods for 'Thunder', the sun, the harvest and a multitude of other things that we now understand and take for granted.
True enough, Lightning can still kill you as sure as if it were the wrath of god, but at least you understand in that final moment that it is electricity that does you in, rather than some upset deity.
It's very hard to imagine a world without science and the progress it has brought us.
Some people say that science has 'gone too far', and that because of science the world is now a dangerous place. Within the same breath you could argue that hammers should be forbidden because you can use those to kill people.
Science is a tool, that can be used and misused, and dependent on how we apply it it can be good or bad that we have mastered so much of it.
Without science, true, no nuclear bombs. But also no radiation therapy. And so on, for every downside there is an upside.
It's very hard to reconcile science with a belief in 'god' or some other overruling super being. However there are areas where scientists have to throw up their hands and say 'we don't know', such as about the 'time' (there was no such thing as time, but that's very hard to imagine so I use that word a little loose here) before the big bang.
Whether or not that confirms the existence of god is another matter, and most scientists - though definitely not all of them - are firmly in the atheist or agnostic camp.
Science has found it's opposite in 'creationism' and other 'pseudo sciences', people that talk suspiciously like scientists do, but without the scientific rigour of proof to hold them back from making outrageous claims.
Usually these people are not pursuing knowledge for its own sake (like most scientists do, even though probably a good portion of them is in it for the cash :), but are pushing an agenda instead under the guise of being scientific.
Some things like 'over unity devices' and 'perpetuum mobiles' as well as the 200 miles-per-gallon carburator are absolutely unscientific hogwash and those who are convinced of their existence and/or selling them are either totally midguided but more usually after your money.