graph

A graph consists of a set of vertices, and a set of edges between vertices.

Formally
A graph $G=(V,E)$ is an ordered pair, where $V=\{v_i:1\le i\le n\}$ is a set of vertices, and $E=\{\{v_i,v_j\}:i,j\in [1,n]\}$ is a set of edges.

The vertices $u$ and $v$ of an edge $\{u,v\}$ are called the edge's endpoints. The edge is said to join $u$ and $v$ and said to be incident on them. When an edge $\{u,v\}$ exists, the vertices $u$ and $v$ are called adjacent.

The order of a graph is the number $|V|$ of vertices.
The size of a graph is its number $|E|$ of edges.
The degree of a vertex is the number of edges that are incident to it; for graphs with loops, a loop is counted twice.

The edges of a graph define a symmetric relation on the vertices called the adjacency relation. Specifically, two vertices $x$ and $y$ are adjacent if $\{x,y\}$ is an edge. A graph of order $n$ is fully determined by its adjacency matrix $A$, which is an $n\times n$ square matrix, with $A_{ij}$ specifying the number of connections from vertex $i$ to vertex $j$.

\documentclass[tikz,margin={3cm 0cm}]{standalone}
\usetikzlibrary{positioning}
%
\begin{document}
\begin{tikzpicture}[node distance=2.5cm]
    \node[draw,circle] (A) {$A$};
    \node[right=of A,draw,circle] (B) {$B$};
    \draw (A) -- (B) node[midway,below=2.5,draw,circle] (C) {$C$};
    \draw (B) -- (C) -- (A);
\end{tikzpicture}
\end{document}