Is stock on the NYSE or NASDAQ normally considered a "liquid asset" on a balance sheet?
Two key components to consider in answering:
(1) How liquid is 'liquid'? Stock might be more liquid that property, but less liquid than bonds. The main reason stock is consider illiquid, is because selling it on short notice may require you to sell during an economic downturn. Overall, a rule of thumb for US equity growth is 7% annually, but in a given year or month, your holdings might still decrease by say 20%.
(2) Shares in what? A share in APPL can be sold in seconds, likely identically to the last posted price. A share in a penny stock might be able to be sold shortly, at a bit of a gap from the last posted price. A share in a private company might take years to sell, if you can even find a buyer at all.
Do you have an example that you're looking at?
GAAP just indicates liquid as "readily able to convert to cash," which includes publicly traded common stock generally; but not private company stock, which is not liquid. This criteria also generally qualifies an asset as a "current asset." Typically, companies will have some rough guideline of the makeup of their current assets. Companies generally hold domestic and/or foreign government debt, foreign currency and various currency related derivatives as current/liquid assets.
It's also not terribly common for public companies to own common stock in other public companies without specific disclosure and generally a broad business relationship, unless the company is an investing business... When you think about it, that makes sense. If I invest in XYZ, the management at XYZ should have a better use for the capital than simply buying shares of ABC to hold in the current assets.