403 - What is Income?

There are certain words that are so commonly used we think they have a universal understanding. One of those words is "income". Think about it, what is income? Your paycheck? Dividends on your investments? Profits from your business? When does an increase on the value of something you own become "income"? The answer to that question is important to more than just the parties in the case Moore v. United States, but to just about every American. Can the United States tax you, as income, for the value of something you have not sold yet?