Q:

What does the plus (+) ornament mean? 

I am learning a baroque piece on the recorder.  Some notes have an unusual ornamentation symbol.  How is a note with a plus sign over it played?  It was suggested to play it as a mordent.  Is that correct? 

A:

Not contradicting @DrSvanHay's source, which actually only arguments, that a trill may be marked as +: This is just indicating any kind of embellishment, I'm not sure, whether due to sloppiness of the composer or copyist, but I frequently find it also in fast movements on very short notes, where a trill will never fit in.  
As example see here in a Boismortier print from 1737:
Note also, that in baroque much more was left to the performer and absence of such a sign does in no way inhibt it.  The word mordent is also not really that stable in its meaning over the course of the centuries, but it may well be the appropriate one in your piece. 
Summarized: either you simply choose an appropriate embellishment depending on the context(what other types of embellishments are present), or you may invest effort to find out, what experts say in that respect or how modern editors rendered the piece in different editions.