Science (from Latin scientia = knowledge) refers to a system of acquiring knowledge based on empiricism, experimentation, and methodological naturalism aimed at finding out the truth. The basic unit of knowledge is the theory, which is a hypothesis that is predictive. The term science also refers to the organized body of knowledge humans have gained by such research.
Most scientists maintain that scientific investigation must adhere to the scientific method, a process for evaluating empirical knowledge under the working assumption of methodological materialism, which explains observable events in nature as a result of natural causes, rejecting supernatural notions. Less formally, the word science often describes any systematic field of study or the knowledge gained from it. Particular specialized studies that make use of empirical methods are often referred to as sciences as well. This article concentrates on the more specific definition.
Science as defined above is sometimes termed pure science to differentiate it from applied science, the application of research to human needs.
Fields of science may also be classified along two major lines:

Experiment, the search for first-hand information, versus theory, the development of models to explain what is observed
Natural science, the study of the natural phenomena, versus social science, the study of human behaviour and society
Mathematics is often referred to as a science, but the fruits of mathematical sciences, known as theorems, are obtained by logical derivations, which presume axiomatic systems rather than a combination of observation and reasoning. Many mathematical methods have fundamental utility in the empirical sciences, of which the fruits are hypotheses and theories.

There are many different conceptions of science.
According to empiricism, scientific theories are objective, empirically testable, and predictive they predict empirical results that can be checked and possibly contradicted.
In contrast, scientific realism defines science in terms of ontology: science attempts to identify phenomena and entities in the environment, their causal powers, the mechanisms through which they exercise those powers, and the sources of those powers in terms of the thing's structure or internal nature.
Even in the empiricist tradition, we must be careful to understand that "prediction" refers to the outcome of an experiment or study, rather than to literally predicting the future. For example, to say, "a paleontologist may make predictions about finding a certain type of dinosaur" is consistent with the empiricist's use of prediction. On the other hand, sciences like geology or meteorology need not be able to make accurate predictions about earthquakes or the weather to qualify as sciences. Empiricist philosopher, Karl Popper also argued that certain verification is impossible and that scientific hypotheses can only be falsified (falsification).
Positivism, a form of empiricism, advocates using science, as defined by empiricism, to govern human affairs. Because of their close affiliation, the terms "positivism" and "empiricism" are often used interchangeably. Both have been subjected to criticisms:
W. V. Quine demonstrated the impossibility of a theory-independent observation language, so the very notion of testing theories with facts is problematic.
Observations are always theory-laden. Thomas Kuhn argued that science always involves "paradigms," sets of (often unstated) assumptions, rules, practices, etc. and that transitions from one paradigm to another generally does not involve verification or falsification of scientific theories. Moreover, he argued that science has not proceeded historically as the steady accumulation of facts, as the empiricist model implies.