Line

--CONTENT --
A line is a real or thoughtful feature that connects two points.

In Geometry
Lines in geometry have names that further characterize them:
The line connecting two points along the shortest path; in Euclid's geometry the line is lengthless and there is only one straight line connecting two points.
segment
ray
side
diagonal line connecting two non-neighborly vertices in a convex polygon;
edge line formed at intersection of two surfaces;
a crossing line
Kryssa The straight line is a partial case of the crochet; other curves are for example:
screw line
ellipsa
parabola
sineoid
hyperbola
cycloside

Etymology
The word "line" is borrowed meaning linen thread, rope.

Other meanings
It follows from the definition of a line that it connects two abstract or physical points.

Abstract matter
As an abstract concept, the meaning of a line is in the sense of:

a family line (which may be a straight line, a Serb line);
Party line;
product line;

a plot line;
time line;
ascending line;
descending line;
minimum resistance line;
power line
a philosophical line, such as:
Plato's line (alogue of the divided line);
For example:
a scientific line;
a line of international cooperation;

urban, extra urban, interurban, international bus/railway, and the urban line may also be a trolleybus, tram or subway line;
a circular line
airline (for aeroplanes);
passenger line;
cruise line;
ferry;
regular/charter line;

contour line (or isoline)
mountain line;
coast/coast/first line;
water line
snowline
horizon line;

Irecole line;
Wallace Line;
a.
a dividing line (e.g. the separation line defined by the Tordesillas contract, the so-called Inter caetera);
border line (e.g. Line Curzon, Line
a demarcation line (e.g. the Green Line in Cyprus);
line of construction, regulation line.

Physical relevance
In the military case, a line is most commonly used in the sense of a defensive line which may be the location of troops (front line) or a pre-built large protective facility such as:
Today's line
Line