Reliability refers to the repeatability of findings. It refers to consistency: if the results of a test or measurement are reliable, a person should receive a similar score if tested on different occasions.. Ex: When people take a vocabulary test two times, their scores on the two occasions should be very similar. If so, the test can then be described as reliable.

Reliability takes the following forms:
1. Test-retest reliability: Similar result obtained when the same test administered twice over a period of time to a group of individuals.
2. Parallel forms reliability: Similar result obtained when different versions of an assessment tool are administered to the same group of individuals.
3. Inter-rater reliability: Similar results are obtained when different judges or raters administer the test.
4. Internal consistency reliability: It is used to evaluate the degree to which different test items that probe the same construct produce similar results.

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Sources:
https://www.uni.edu/chfasoa/reliabilityandvalidity.htm
http://psychology.ucdavis.edu/faculty_sites/sommerb/sommerdemo/intro/validity.htm