An exact science constructs theories making predictions which can be verified or falsified using quantitative and rigorous measurements. Usually the natural sciences are considered to be exact sciences, while social sciences are considered to be non-quantifiable. More colloquial terms going among similar lines are 'hard science' and 'soft science'. It is important to note that the existence of a theory which is non quantifiable right now in any science, does not imply this science to be not exact. Several important theories existed for long times until experiments (or measurements) could be designed to test them. Some sciences have or had components of hard and soft sciences. Sometimes new measurement methods and technologies can change a soft science into a hard one (e.g. medical science over the last few hundred years).
Sciences which aim to fulfill the ideal of an exact science and assume Naturalism are mathematics, natural sciences (physics, astronomy, chemistry, biology), Geosciences (geology, meteorology etc...) and engineering science. Sciences which aim to use exact scientific methods without requiring Naturalism exist, for example creation science and baraminology.
Sciences which do seldom aim to be make exact predictions are social sciences, and linguistic sciences. However in the form of statistics and simulations exact methods also enter these fields.