I don't think there's a strict mathematical definition. A straight line is one of those elementary geometric objects (along with terms like point, angle, distance, etc) that are assumed to exist, out of which other figures may be constructed.
You could say "shortest distance between points," but that doesn't necessarily convey the notion of "straight" - especially in non-euclidean geometries where geodesics may be curved - nor imply the (usually) infinite extent of a line. Without actually defining "line," we agree that if all the points belong to the same line then they are co-linear, which we then call "straight." At least that's how I think it works.
Algebracially you can define a straight line in terms of relations among the coordinates of each point. Of course you first have to establish coordinate axes, which are all assumed to be straight lines. So that approach seems shaky, too, lol.
In the physical world we can define "straight" as "the path that light takes," though it's still a slippery concept with much to quibble about.
Yes, it has to be straight or it is not a line.
yes verticle is up and down
oblong
Yes. If it is not straight, then it is not a line.
Displacement of a straight line is zero...
A straight line has no vertex.
it just a straight line
If it is not straight, then it is not a line.
There are 180 degrees on a straight line.
The word straight does have an antonym, which is crooked. So perhaps a crooked line is the opposite of a straight line.
The length (distance) of a straight line always changes, the angle of a straight line is 180 degrees.
The straight-line distance is 1,030 miles (to the coastline of Puerto Rico). This is the true straight line distance which accounts for the curvature of the earth, NOT simply a straight line drawn on a map.
A line that goes straight acrosswould be ahorizontal line.
A straight line is a line with the property that, if you pick any two points on the line and connect these points with a straight line, then every point on this new line lies on the original line.
Straight line.