A vector is a collection of elements whose order matters. If the elements represent comparable measurements it can be said that they together represent a direction and a magnitude (length). The Pythagorean theorem is used to find the length of a vector.
This is to be represented to high school students. I want to ask for opinions so the definition can be as useful as possible while not misleading them.
EDIT: forgot to mention that the students are familiar with what a set is.