N.Y. Comp. Codes R. & Regs. tit. 1, § 188.4 – Solidity classification.

(a) Hard means that the head is compact and solid. This term represents the highest degree of solidity.
(b) Firm means that the head is compact, but may yield slightly to moderate pressure.
(c) Fairly firm means that although the head is not firm, it is not soft and spongy, and has good head formation and edible content.
(d) Soft means that the head is easily compressed or spongy.

The following terms shall be used in describing the solidity of lettuce: