Here is a search string to return studies in PubMed:
"Cohort Studies"[Mesh] OR "cohort study" OR "cohort studies" OR "follow-up study" OR "follow-up studies"OR longitudinal OR "prospective study" OR "prospective studies" OR "retrospective study" OR "retrospective studies"
This hedge is from the University of Texas https://libguides.sph.uth.tmc.edu/search_filters/pubmed_filters is very similar except that it limits the keywords to [tiab] and lists the MeSH terms included as subcategories under "Cohort Studies"[MeSH]
PubMed: Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics.https://www-ncbi-nlm-nih-gov.ezproxy.rush.edu/mesh/?term=cohort+studies
Wikipedia: Cohort studies differ from clinical trials in that no intervention, treatment, or exposure is administered to participants in a cohort design; and no control group is defined. Rather, cohort studies are largely about the life histories of segments of populations, and the individual people who constitute these segments https://en.wikipedia.org/wiki/Cohort_study
Sum it up: Cohort studies are observational studies in which two groups (or arms, or cohorts) are observed. Often (always?) prospective, not retrospective, and sometimes studies are labelled "prospective cohort studies."
Here are some examples of articles tagged with the MeSH term for Cohort studies:
You may want to ask your researchers if they would like you to also search for retrospective studies. Retrospective studies are a type of cohort study, but should therefore be caught with the MeSH term Cohort studies. But it might be worth searching on the keyword "retrospective" as well.