The term PART OF MST can be broken down into two crossword solutions: ESS (3 letters) and STD (3 letters). ESS stands for Elementary School System, which is a system of public schools in the United States that provides primary education to children from kindergarten through fifth grade. STD stands for Standard Time Difference, which is the difference between two time zones or the amount of time one must add or subtract from the local time to obtain the standard time.