If you are not familiar with catalogue search basics you should view the Library Skills Tutorials. Here are a few search tips for finding items related to medieval history.
Medieval tends to refer to the broader period while Middle Ages can be broken down into Early, High or Late Middle Ages. Mediaeval is a lesser used spelling. To make sure you retrieve as many catalogue records as possible you want to take this into account. A keyword search could contain the following, but it would probably retrieve far too many results.
'middle ages' or medieval or mediaeval
It would be better to combine the above search with other search terms to help you find topic ideas. Remember, if combining a search using different boolean operators (AND, OR, NOT) it is best to put multiple search terms associated with one operator in their own brackets. This way, the Library Catalogue will search those terms first and then combine them with the other search term. Remember, in the Library Catalogue $ is the truncation symbol. In the following example the Library Catalogue will retrieve records with war, wars, warrior, warriors, warring.
('middle ages' or medieval or mediaeval) and war$
('middle ages' or medieval or mediaeval) and (religio$ or church or monast$ or liturgy or hermit or hermits or recluse or recluses)
Once you do a search and get a results list in the catalogue quickly examine the list and look for items that appear to be on your topic. Look at the full bibliographic record and see if there are subject terms that you can use to help refine or expand your search. For example the following are useful terms that cataloguers use when creating records for books that cover economic or social history:
You can use this knowledge to create searches like ('middle ages' or medieval or mediaeval) and ('economic conditions' or 'social conditions')
Searching the catalogue using historical figures as search terms is very useful. Thus, you need a basic grasp of the historical events and persons. Use your textbook, subject specific reference books, websites, etc. to find such terms. For example, Constantine the Great, also known as Constantine I or Saint Constantine, was Roman Emperor from 306 to 337. A catalogue keyword search using 'constantine the great' or 'constantine I' or 'saint constantine' retrieves more items compared to using only the search term 'constantine the great'.
You can search the catalogue by specific historical events. For example, the Crusades were a significant part of Medieval history. Doing a keyword search for crusades will retrieve items, and because they were most associated with the Medieval period you do not need to use this or its synonyms as search terms.