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Length is the space inside a one-dimensional segment or curve
Giving a formal definition of length is tricky. Informally, length measures "how long" a one-dimensional object (segment or curve) is, in miles, meters, feet, or some other length unit. You can also think of length as measuring "how far" the endpoints of the object are from one another. Or "how much space" there is between the endpoints.
You can visualize length and related concepts using a number line, which is basically an abstract one-dimensional ruler.
Relationship between length and distance
Length and distance are intimately related concepts both having to do with line segments. The length of a line segment is the distance between the two endpoints of that line segment (and vice versa).