Cotyledon: A primary leaf of the embryo; a seed leaf.*
This cauliflower seedling has two cotyledons. Cotyledons are not true leaves, they are part of the original embryo inside the seed. You can see the first true leaves of this seedling just beginning to develop in the centre of the plant, between the two cotyledons. Cauliflower, like other brassicas, has heart-shaped cotyledons which are very different in shape from the true leaves.
*Harris, J.G., Harris, M.W. (2001). Plant identification terminology: An illustrated glossary (2nd ed.). Spring Lake: Spring Lake Publishing.