Watts defines an F1 as:
the heterozygous offspring between two homozygous but unrelated seedlines. This makes sense and gives the F1 generation a unique combination of traits; uniform phenotype but not true breeding. This is important in the plant breeding world. This means that when a customer buys F1 seeds that they should expect uniform results.
source- Watts. 1980. Flower & Vegetable Plant Breeding Grower Books, London