"a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe"I would like to tighten that definition up slightly. I would argue (perhaps controversially) that science is a method of testing and explaining how the universe works, in the current moment. Science is fundamentally based on a (usually unstated) presupposition that the way the universe works now is the same as the way it has always worked. "Ye canna change the laws o' physics" as a great man once said. Or, more importantly, the laws of physics don't change. (Whether an agent can change them is a discussion for another day.)(from Wikipedia, paraphrasing Webster's Dictionary)
When I do experiments in the lab, I assume that, given the same environmental conditions, the experiment would work in exactly the same way in Beijing as in Edinburgh, and in exactly the same way in 1812 as in 2012. Experience bears this out. When we find discrepancies between experiments carried out in one place and time from results obtained in another place and time, we always find that it is the environmental conditions that are different, not the laws of physics.
Fundamentally, science is the study of the ways things work now, assuming that this will not change in the future and would have been the same in the past.
Science works by proposing a hypothesis (or a range of hypotheses), then carrying out experiments or observations to collect data, which can be used to either confirm or refute the hypothesis.
If a hypothesis has gone through this cycle of experimentation and analysis sufficiently many times, has not been refuted but has been confirmed many times over, then the hypothesis gets upgraded to the rank of 'Theory'. Like the theory of evolution or the theory of gravitation.
In common parlance (particularly in the minds of anti-evolutionists) there is no distinction between a hypothesis and a theory. "It is only a theory" is used to scoff at evolution. The phrase "It is only an experimentally validated hypothesis" should be equivalent, yet this doesn't sound so dismissive, so isn't generally used...
When we come to evolution, we have a problem as far as science goes. In an ideal world we'd simply set up an experiment, wait a couple of hundred thousand years, then examine the data. If we could do that, I have little doubt that the theory of evolution would be validated. Then, evolution would be what I can real science - we'd have proof that evolution is how the world works now.
In the absence of that data (which our descendents will get eventually!) we have to rely on observations rather than experiments. (OK, yes, we can do experiments with bacteria and the like and observe 'micro-evolution' over many hundreds of generations, but we still don't have the timescale to observe 'macro-evolution' whereby those bacteria evolve into non-bacteria.) We can observe the fossil record and we can study the DNA of living (and preserved) animals and make inferences from that. In essence, we can construct an 'experiment' where we take the evidence at the start of the trial (i.e. fossils from a particular early stratum) and compare these with evidence midway through the experiment (i.e. fossils in a more recent stratum) and also with evidence at the end of the experiment (i.e. animals now) and see if the evidence supports or refutes the theory. Much of the evidence examined in this way does indeed support the theory of evolution.
Thus, from a scientific viewpoint, the theory of evolution is validated. Micro-evolution is observed at the micro-scale, and has been shown to have predictive capabilities - predictions have been made regarding bacteria evolution (or adaptation, if you would rather) and these have been demonstrated by experiment.
But evolution only goes so far. It is, by definition, a theory which shows how a population of organisms can change over many thousands of generations. What it can't do is explain how the original population came to be. This is where ID likes to jump in.
The basic point of ID is that because science has no explanation how the original population of organisms came to be, it is plausible to suppose that perhaps an Intelligent Designer started the whole ball rolling. To be honest, I don't have an issue with that line of reasoning, but it is philosophy, not science.
The ID hypothesis has no predictive power. It tells us nothing about how organisms in the present day will react, adapt, evolve, or otherwise change with subsequent generations. Given this, it is not and should not be regarded as science.
ID also pre-supposes the actions of an agent from outside of science effecting change inside science. This basically constitutes a discontinuity in reality, whereby we must (if we accept ID) assume that science changed at the time of the action. In other words, there was a change in the laws of physics. Once again, this is not science, based on the fundamental assumptions of what science is.
ID should only be discussed in the philosophy class, not the science class.