There is no legal definition of "secondary employment." Rather, every employer can set its own terms and conditions for whether employees may freelance, moonlight, have a second job, etc. Unless you have a contract to the contrary, one guaranteeing you the right to other work or restricting the grounds on which you could be disciplined or terminated, your employer could elect to discripline or fire you for having other employment, even if it does not directly impact your employer in any way. Or they could put other restrictions on you, such as requiring you to let them know of your extra-curriculuar doings. It may be silly and it may be unfair, but it is legal--an employer could make no outside employment a term or condition of employment.