f2 hybrids are a cross between two f1 hybrids, and are not considered stable.

Definition as written by Terry:
The second filial generation; hybrids resulting from self-pollination within a population of F1 hybrid plants - they will not come true to form.

Definition as written by Margiempv:
Second generation resulting from self-pollinating or sib-crossing the F1 seedlings.

Definition as written by Magpye:
This is the product of two F1 plants that have been crossed. This is considered the second generation. This will not necessarily produce a great plant.