Complementizer deletion

In generative syntax, a complementizer deletion is the process of deleting a complementizer.

Examples
(i)  a I know that he is here b I know _ he is here

This deletion has also been taken to apply to wh-words in relative clauses as in (ii).

(ii) a the man whom I have seen b the man _ I have seen

Comments
The underlying assumption is that the wh-word is in the COMP position. This assumption has been abandoned with the introduction of CP. Now the empty slot '_' in (ii)b is usually analyzed as an empty operator.

Link
Utrecht Lexicon of Linguistics