The term 'cornerstone abbreviation' refers to two abbreviations commonly used in the establishment of a business or organization. The first abbreviation is ESTAB, which stands for 'ESTABLISHED'. This is typically used to indicate when a business or organization was founded. The second abbreviation is ESTD, which stands for 'ESTABLISHED Date'. This is typically used to indicate the date on which a business or organization was founded. Both of these abbreviations are commonly used in legal documents and other official records.