Returns the value specified in sourceField using the relationships in the relationships graph. The result of the optional failExpression will be returned if the lookup fails.
sourceField - the field from which the lookup value is taken.
failExpression - any expression.
Parameters in curly braces { } are optional.
Data type returned 
text, number, date, time, timestamp, container
Originated in 
FileMaker Pro 7.0
For this function to access the contents of the source field, the tables containing the source field and calculation field need to be related. Calculations using the Lookup function won’t be forced to be unstored calculations.
Note  Lookup returns ? when the related table is an ODBC data source.
There are two tables, People and Company, in a database file containing the data shown below.
People table
Company table
The People and Company tables are related using the number field CompanyID. The calculation CompanyName = Lookup(Company::CompanyName;“Not found”) defined in the People table will return Apple for the first record, FileMaker for the second record, and Not found for the third record.
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas