Extra dimensions refer to the theoretical concept in physics that there may be additional spatial dimensions beyond the three dimensions that we are familiar with (length, width, and height). These extra dimensions are proposed in some theoretical models, such as string theory, in order to explain various phenomena that cannot be accounted for by the standard model of particle physics. The existence of extra dimensions could potentially help explain the hierarchy problem, unify the fundamental forces of nature, and provide a deeper understanding of the nature of spacetime. Research in this area involves theoretical modeling, experimental testing, and seeking observational evidence for the presence of extra dimensions in the universe.