Rensselaer Polytechnic Institute had a patent for the technology which was granted in 2007. Another outfit called Dynamic Advances holds the exclusive license to it and sued Apple in 2012.

Now it seems that Apple has given up defending this particular case and has written a $24.9 million check to make Dynamic Advances go away.
In exchange for $24.9 million, Apple will be allowed to continue loading its devices with Siri and the assurance that it's not going to be sued based on the same patent again... at least for the next three years.

Dynamic Advances is getting $5 million as soon as the case is dropped, with the rest to follow. It expects to pocket half of the amount and divvy up the rest to pay Rensselaer and its lawyers, among the other entities involved in the case.

The patent was for a "Natural language interface using constrained intermediate dictionary of results." The document says the invention "relates to user interfaces, and more specifically, to user interfaces that recognise natural language."