Fathy's answer is good.
However, a lexical field is not a semantic field. A semantic field corresponds to the semantic options (concepts) you have for a particular slot in a clause while a lexical field corresponds to the lexical options (terms) you have for the same slot. For instance, one person may say "yesterday I met my nanny" while another person may say "yesterday I met my grandma". These clauses represent identical events, that is, they are semantically equivalent, but they represent the identical events with different terms, namely "nanny" versus "grandma".
One way of describing English consists of assuming that "nanny" and "grandma" are different terms associated with the same concept. Since these are different terms, they are two options within the same lexical field. However, since they are associated with a single concept, they are associated with one option in a semantic field that includes "grandpa", "mother", "father" etc..
I hope this helps. -- Daniel