Lexical accuracy is based on the proper use of the words with their correct meaning, so to be understood by the person receiving the message that you are communicating. In addition, the words should be used without intent to Double Entendre or ambiguity, or errors of any kind ( Ortograficos or pronunciation ).
Pointing in the same direction as Ines Merino, it's expressive accuracy of spoken or written language. And it is true, as says our Ines, that when some express something, make it so well that it is better to remain silent.
Precision means realization of something that joined the lexical Word, comes to refer to when we say the exact words for the specific time or the perfect phrase. Sometimes, gets much the leg with what is said and does not match with the words coming to emit unwanted erroneous messages, so we must think before you speak to avoid unnecessary and uncomfortable situations.