>>> As a noun, it's a second helping of a dish (not the vessel itself, can't help that, but what's poured inside) during a meal.
>>Excuse the pedantry, but In the example you used, it's an adjective.
>
>Perhaps you are right... I thought it was another adjective used as a noun, as in military, musical, western. But now that you made me think of it, yes, it's still an adjective. The evolution still hasn't run its full course (or second course, for that matter...).

It's a noun in - "I'll be there in a second."

In the phrase you used it's an ordinal - first, second, third, etc. They are usually adjectives, but sometimes they imply the noun they modify, as in the classic "Who's on first?"