It varies, but "Assoc. Prof." is the most widely used abbreviation. When in doubt, a decent rule of thumb is to spell it out: "Associate Professor." It is allowed to write "Professor Jones" at all professorial levels in less formal writing (e.g., a student emailing a professor). In academic contexts "Dr. Jones" is preferred. An associate professor has a faculty rank between that of professor and full professor. Although they are expected to have significant teaching responsibilities, associate professors can also conduct important research. Generally, someone must make the recommendation for an applicant to be appointed as an associate professor. The process typically includes a search by the department or school for the best candidate, followed by approval by the university administration. A candidate may also be appointed as an associate professor without going through this process if they have a distinguished record of academic achievement and their appointment is supported by the dean or director. Generally, people use their full names when writing to an associate professor. However, some universities have different rules for what degree to use after a person's name. If a university uses "Mr./Ms." as their default form of address for everyone except professors, then you should use "Mr./Ms." too. Otherwise, you might come across as disrepectful to the other members of the faculty. Use your best judgment here; if in doubt, use "Mr./Ms."