Final answer:
A Computed column is a virtual column derived from an expression that uses other columns in a table and is not physically stored unless marked as PERSISTED.
Step-by-step explanation:
The term in the question that refers to a value that does not physically exist within the base table but is calculated from other columns is a Computed column. A computed column is a virtual column that is not physically stored in the table, unless the column is marked PERSISTED. It is created by a mathematical expression that uses other columns in the table. For example, if a table has columns 'price' and 'quantity', a computed column could be 'total_cost' which multiplies 'price' by 'quantity'. This is in contrast to a Primary key, which is a unique identifier for a table row, a Foreign key, which is a reference to a primary key in another table and used to maintain referential integrity, and an Index, which is used to speed up the retrieval of data from a database table.