2020 Mathematics Subject Classification: Primary: 05C99 [MSN][ZBL]
An unoriented graph in which each vertex has the same degree. If the common degree is $k$, the graph may be termed $k$-regular.
A strongly regular graph is a regular graph in which any two adjacent vertices have the same number of neighbours in common, and any two non-adjacent vertices have the same number of neighbours in common. The complement of a strongly regular graph is again strongly regular.
A distance regular graph is one with the property that for any two vertices $x,y$ the number of vertices at distance $i$ from $x$ and $j$ from $y$ depends only on $i$, $j$ and the distance $d(x,y)$.