The word side refers to the segments that make up a polygon (like a triangle or a square).
The word "side" becomes ambiguous when we are talking about three-dimensional figures. For example, casual (but not mathematical) speech might describe a cube as having six "sides," yet when people are asked how many "sides" a room (shaped exactly like the cube) has, they tend to answer four (not counting the ceiling or floor as "sides").
Also, if the "sides" of a square mean the line segments that surround it, we might be tempted to say that a cube has twelve "sides" -- the twelve line segments that surround the square faces of the cube.
So, a question like "How many 'sides' does a cube have?" is ambiguous, because the answer might be four, or six, or twelve, depending on what "sides" is taken to mean. Because clarity and lack of ambiguity are so important in mathematics, mathematics does not use the word "side" for flat-surfaced three-dimensional figures, preferring face for polygons that form the surface of the figure, and edge for the line segments at which any two faces meet.