A scale drawing is a drawing where the dimensions are proportional
to the actual size of the object being drawn in a predetermined ratio.
Scale drawing is an application of the concept of similarity.

 In plain English, a scale drawing is a drawing which has been reduced or enlarged from its original size, to a specified scale. (Defined by Collins English Dictionary). The scale is a ratio of the size of the drawing to the size of the original object being drawn. This may be referred to as a scale ratio. It compares the scale drawing of an object to its actual size. For example, if the scale is 1 cm : 3 cm, then the length of 1 cm in the drawing represents 3 cm in true size (or original size).
The term scale factor is the ratio of a measurement of the drawing compared to
the corresponding measurement of the original figure.
A scale factor is a number used as a multiplier in scaling.
(Units are not mentioned in a scale factor.)

Example: A sidewalk 20 feet long is drawn on a map as 4 inches in length.
The scale factor of 1/60 tells us that the map is 1/60 times the size of the actual sidewalk.

 If the scale factor is < 1, the created object is smaller. (reduce) If the scale factor is > 1, the created object is larger. (enlarge)

Remember that the scale factor is the number you multiply
times the initial object's measurements to get the new, created object.

Scale drawings are used in a variety of fields: art, cartooning, design, advertising,
construction, architecture, floor plans, mapping, geography, landscape, cellular biology, geometry, physics, astronomy, engineering, chemistry, aerospace, archaeology,
and the list goes on.

 A scale replica of the starship Enterprise (from Star Trek) has a length of 8 inches. The actual starship is said to be 2,108 feet in length. What was the scale ratio used to build the replica?
 Solution:

 During WWII, soldiers drew pictures on the noses of their aircrafts, which were called "nose art". A small initial drawing was made, and then enlarged onto the plane. If the scale factor of the enlargement was 12/1, and the length of the initial dog was 6 inches, what was the length of the plane's bulldog in inches?
 Solution: