What are the combinations of "**there-is**" or "**there-are**" called in English grammar terms?  

> For example: **There is** an apple on the table.

I am looking for a specific term for these expressions (as well as for the same in past tense such as "**there was**" and "**there were**"). 

Normally in many of languages, unlike in English these two combinations are expressed in one specific word.