Operation that assigns to every pair $$\left( \overrightarrow{u},\overrightarrow{v}\right)$$ of vectors a new vector $$\left(\overrightarrow{u},\overrightarrow{v}\right)$$ called the vector sum or the resultant vector.
Therefore, if $$\overrightarrow{\textrm{AB}}$$ and $$\overrightarrow{\textrm{BC}}$$ are two vectors, then $$\overrightarrow{\textrm{AB}}+\overrightarrow{\textrm{BC}}=\overrightarrow{\textrm{AC}}$$. This equality is also called the Chasles relation.