Q. What exactly is meant by the word equivalent in the phrase "four-year degree or equivalent"?
A. When an employer uses the term equivalent, it typically means they will consider any job-related experience that will provide the candidate the necessary knowledge, skill, and abilities (abbreviated "KSA") to be able to perform the functions of the advertised position proficiently. A formal education with a four-year degree can be sufficient to provide a candidate with necessary KSAs. Or, a high school diploma and four years of experience in a particular job can also help a candidate develop sufficient KSAs. A company is simply looking for someone who has the KSAs, whether from an informal background or through a formal avenue, to perform the functions of a job successfully.
The same logic applies when an employer says it will consider "two years of equivalent experience." In this case, the employer is looking for either a formal education or job experience that gives you the knowledge, skills, and abilities.