The term 'cornerstone abbr.' refers to three abbreviations commonly used in the English language. The first is ESTAB, which stands for 'ESTABLISHED'. This is often used to refer to a business or organization that has been in existence for a certain amount of time. The second is EST, which stands for 'ESTABLISHED' and is often used to refer to a date or event that has been ESTABLISHED. The third is ESTD, which stands for 'ESTABLISHED date' and is used to refer to a specific date when something was ESTABLISHED. All three of these abbreviations are commonly used to refer to the founding or establishment of something.