Just to add to this, it is commonly used as the first generation from wild caught parents. But really it could be from any parental breeding. Here is the definition:
(genetics) The first generation resulting from a cross between parents homozygous for different alleles at a locus; all members are heterozygous at the locus. Symbolized F1.
This was actually discussed recently in another post. You may want to check that one out also.
Jim G