MSDN Reference
Optimizer hints that effect the entire query - not specific to any 1 part of the query like a table hint.
The OPTION clause specifies the hint:
SELECT * FROM Sales.Customer AS c
INNER JOIN Sales.CustomerAddress AS ca
ON c.CustomerID = ca.CustomerID
WHERE TerritoryID = 5
OPTION (MERGE JOIN);
Hints such as:
MAXDOP
RECOMPILE
KEEP PLAN
FAST number_rows
FORCE ORDER (of tables as they are given in join clauses)