Look-ahead

'Look-ahead' is an intuitive term for the phenomenon that certain grammatical principles filter out derivations which under local economy can only be filtered out by looking ahead to a later stage in the derivation. Instances of look-ahead in Chomsky (1995) are: - determining which features are uninterpretable at the interfaces and hence must be deleted (by either movement or merger of an element with matching features) - determining which element projects when two elements are merged - global economy principles like Procrastinate Chomsky (1998) tries to do away with look-ahead by applying Spell-Out and LF-interpretation cyclically to phases.

Example
In general, Procrastinate bars overt movement to weak checking positions. In some cases, however, Procrastinate can be violated:

(i) What did you see?

In (i) what is moved from a weak position (case in vP) to a strong position (Q in CP). Overt movement to weak checking positions can only occur when it is followed by movement to a strong checking position, and so it seems like you have to look ahead in the derivation to determine whether overt movement to a weak position is allowed.

Link
Utrecht Lexicon of Linguistics