A vector is a quantity with both magnitude and direction, which can be represented by a line segment with an arrow. The length of the line segment indicates the size of the vector, and the direction pointed by the arrow indicates the direction of the vector. The size of the vector can be positive, negative or zero, and the direction can be any direction. In mathematics, vectors are usually represented by lowercase bold letters, the size of vectors is represented by the length or modulus of vectors, and the direction of vectors is represented by unit vectors. Vector is widely used in the fields of mathematics, physics and engineering, and can be used to describe the change of physical quantities, the movement and geometric shape of objects. Vector operations include vector addition, subtraction, number multiplication, point multiplication and cross multiplication.