Medievalists as a rule cannot agree upon any firm grounding for what the period is. It also varies depending upon what area is being discussed and what the departmental organization and interests of those discussing the matter are. Medieval Studies, as a field, has generally and historically focused on European history (largely Western Europe) during the period between the fall of the (Western) Roman Empire and the beginning of the (early?) modern period. Early Modern history is generally treated as a separate specialty, but significantly complicating the picture are two related fields, Late Antiquity and Renaissance Studies, which are sometimes part of medieval studies and sometimes part of Ancient History or Early Modern Studies (respectively). Just to show how confusing this is, Peter Brown, widely considered one of the preeminent medievalists, is a medieval-trained Late Antiquity specialist, famous for his work on Augustine of Hippo, a man that lived his entire life in the period before the fall of (western) Rome and could easily be departmentalized in Ancient History or Early Christian Studies.

The Renaissance, insofar as anyone agrees whether that is a real and meaningful term, may be a separate field of study/departmental specialty or part of medieval studies. Problematically, there are many periods that have been labelled 'renaissance' at this point, many explicitly in even the narrowest remit of medieval studies, including the Carolingian Renaissance, the Ottonian Renaissance, and the Renaissance of the long twelfth century; divergence begins around the Italian Renaissance, the Spanish Renaissance, and the English Renaissance (etc.). All of these may be part of the field of medieval studies (e.g. Dante scholarship is presented at the general medieval conferences) or part of a separate department (at my doctoral institution, 'Medieval Studies' and 'Renaissance and Reformation Studies' were separate units).

Various temporal borders for the field have been proposed, and it is often necessary to delineate the scope of courses, such that 500-1500 is a common time frame (the period covered by The New Cambridge Medieval History), or from the fall of the Western Roman Empire (476) to the fall of Constantinople (1453). Textbooks and journals give a clue to the disagreement: Blockmans & Hoppenbrouwers as well as Rosenwein and Riddle & Black all cover 300-1500; Holmes covers "Europe between the fall of the Roman Empire and the coming of the Renaissance" (see above for the issues with that); Speculum, a flagship journal, covers approx. 500-1500. The dates can also be very regional: in English history the medieval period usually ends in 1485 when the War of the Roses concludes and the Tudor-Stuart period begins. In the latter half of the 20th century, as medieval studies expanded its scope and started to encompass the Mediterranean, these regional differences in what constitutes medieval have only grown more problematic with the wider range of cultural groups involved (e.g. Muslim history talks about the pre-Islamic period, rather than being focused on Rome). As a broader, 'global' medieval studies begins to take shape, it is even more unclear where the delineation is, and how responsive it is to local periodization and historiography. The most coherent and consistent argument I have seen for the end of the medieval period is that it ends when the Columbian exchange begins, that is to say, in 1492, when both hemispheres were brought into consistent contact and the import of goods and people (and people as goods) began.

For further reading, check out the sources in footnotes 1. and 2. of Hutton, Medieval or Early Modern https://www.cambridgescholars.com/download/sample/62502

For an argument about why the Columbian Exchange is the end of the Middle Ages and why the MA do not include the Americas, see Alex West, "The Middle Ages" https://medium.com/@IndoMedieval/the-middle-ages-86a7a261c1f5 and "The Hemispheric Middle Ages---Part I" https://medium.com/@IndoMedieval/the-hemispheric-middle-ages-part-i-173779f237f6