What is the word that means there is a difference in import of the same words when the exact same words are spoken by a different person

I have come across this word in a book; it gives the example:

If a layman says “I now pronounce you man and wife,” it doesn't make the couple husband and wife.

But when the same words are spoken by a priest, it makes them husband and wife.

The word possibly starts with either "i" or "l".

Solution 1:

In reference to a comment posted by Shoe and a comment posted by Edwin Ashworth, the phrase you are looking for is Illocutionary acts.

This was defined by John L. Austin as follows:

I explained the performance of an act in this new and second sense as the performance of an 'illocutionary' act, i.e. the performance of an act in saying something as opposed to the performance of an act of saying something — J. L. Austin, ed. J. O. Ursom, How to Do Things with Words, 2011, Clarendon Press, Oxford.