Why is psychology considered a science?

Science, in it's purest form, is defined by the use of clear, substantiated methodologies to obtain an informed understanding of something in the world around us. In the case of psychology, that "something" is human behavior and it's numerous idiosyncrasies. As the previous poster elucidated before, science often entails the use of the scientific method to confirm or refute hypotheses. As with other scientific disciplines like biology or chemistry, research in the psychological sciences involves utilizing a specific study approach (experimental, case study, longitudinal study, double-blind, single-blind) to obtain and analyze data (through intensive observation, MRI, neuroimaging devices, etc.) that materializes through studies. Researchers in the field also often employ statistical analyses to examine trends that their data reveal, much like a biologist or chemist may do. From there, they can determine whether their hypotheses are sufficiently supported or unfounded.
These processes are repeated several times in many cases, depending on the validity of the data, researcher biases and proposed implications of said research. Just as a marine biologist might want to extend their research parameters in terms of which variables (time of day, presence of predators nearby, etc.) influence mating behaviors in crustaceans, a social psychologist might seek to reexamine the variables that contribute to a phenomenon such as diffusion of responsibility, like individual temperament or personality. In both cases, biases are being examined and extraneous variables are being considered, which is a crucial step to expanding scientific inquiry.
In some cases, you might hear people differentiate psychology as a "soft science", as it examines seemingly commonplace and easily observable phenomena (not always of course). These same people will argue that the methodologies used aren't as rigorous or invasive enough to be considered legitimate. While these criticisms might seem disheartening to some, the truth of the matter is that research in psychology is held to the same level of scrutiny and review as in any other science (meeting IRB standards for research, obtaining funding, etc.). So long as psychological research continues to abide by the standards by which it currently operates, it will remain a legitimate, credible scientific discipline.
Sources:
1.) https://sciencecouncil.org/about-science/our-definition-of-science/
2.) Past undergraduate coursework and experiences (speaking with other students and professors in different scientific disciplines).

You are probably familiar with the scientific method, in which one forms a hypothesis and tests it. In order to determine whether a hypothesis is correct, the outcome must be the same no matter how many times one performs the experiment. For example, zoologists might test how a specific type of algae affects the mating habits of a specific type of bird. The researcher might repeat the same experiment over a decade or more to be sure the outcomes match up each mating season. When the researcher has enough data to support his/her claims, the research becomes part of a body of knowledge in the field of zoology, specific to the species of bird, and has cross-disciplinary applications in botany (algae) and climatology.
Psychological research works very much in the same way. Psychologists approach an issue with a hypothesis. Researchers apply the scientific method. Researchers repeat their experiments several times in order to predict outcomes. Researchers present their findings to peers within their discipline.
An example would be studying the best approaches for treating PTSD in combat veterans. The hypothesis is that pairing a veteran with a service animal and prescribing pharmaceuticals has a higher success rate than pharmaceuticals alone. The researcher would have participants assigned to each scenario, and then draw conclusions based on outcomes. Those outcomes can then inform the procedures of psychologists, psychiatrists, social workers, and the Department of Defense.