The term 'sideline' can be defined by four crossword solutions: BORDER (6 letters), AVOCATION (9 letters), HOBBY (5 letters), and IDLE (4 letters). A BORDER is a line that marks the outer limits of an area, while an AVOCATION is an activity done in one's spare time for pleasure. A HOBBY is a pastime or activity that someone does for enjoyment, and IDLE means to be inactive or not doing anything. Together, these words can be used to describe the term sideline, which is an activity done in one's spare time that is separate from one's main job or occupation.