is a linear function because It forms a straight line or represents the equation for the straight line
It has only one degree. Or we can also define it as an equation having the maximum degree 1.
All these equations form a straight line in XY plane. These lines can be extended to any direction but in a straight form.
The general representation of linear equation is;
y = mx +c
Where x and y are the variables, m is the slope of the line and c is a constant value.