Calyx: all of the sepals (floral leaves) of a flower, which form the protective outer layer of a flower bud. (See http://www.rbgkew.org.uk/ksheets/pdfs/flower.pdf for a fact sheet on flower structure).
Sepal: a floral leaf (collectively comprising the calyx of the flower) that forms the protective outer layer of a flower bud. (See http://www.rbgkew.org.uk/ksheets/pdfs/flower.pdf for a fact sheet on flower structure).