K_4 is a complete graph in graph theory, consisting of four vertices, where every pair of distinct vertices is connected by a unique edge. This means that there are a total of six edges in K_4, making it a simple yet fully connected structure. In terms of its properties, K_4 is a planar graph, meaning it can be drawn on a flat surface without any edges crossing. It also serves as an important example in various areas of mathematics, including combinatorics and topology, illustrating concepts such as connectivity and graph coloring.