leaf lettuce

Any of several varieties of lettuce with leaves that branch from a single stalk in a loose bunch rather than forming a tight head. The leaves are crisper and more full-flavored than those of the head lettuce varieties. Depending on the variety, leaf lettuce (also called looseleaf and Simpson lettuce) can range in color from medium to dark green; some have red-tipped leaves. Among the more popular leaf lettuces are oak leaf, salad bowl, frilly red leaf and crinkly green leaf. In general, leaf lettuce is more perishable than head lettuce. Choose bunches with crisp, evenly colored leaves with no sign of wilting or yellowing. As with all greens, leaf lettuce should be washed and either drained completely or blotted with a paper towel to remove any excess moisture before being refrigerated in a plastic bag. It will keep this way up to about 3 days. See also lettuce.

The noun has 2 meanings:

Meaning #1: distinguished by leaves having curled or incised leaves forming a loose rosette that does not develop into a compact head
  Synonym: Lactuca sativa crispa

Meaning #2: lettuce with loosely curled leaves that do not form a compact head
  Synonym: loose-leaf lettuce