If tax is applicable on a the profit derived from a sale of an asset/security/investment, it is called taxable gain.
Taxable gain is the difference between the selling price of a security and purchase price.
I bought shares at 10$. Total shares bought were 1000. After let us say 5 months, the price of one share was 20$ and i sold all 1000.
My gain would be 1000(20-10)=10000$.
This is now taxable.
For gain to be taxable, the shares or securities actually needed to be sold in the market. Without actual sale, the concept does not hold.