Notional amount
The notional amount (or notional principal amount or notional value) on a financial instrument is the nominal or face amount that is used to calculate payments made on that instrument. This amount generally does not change hands and is thus referred to as notional.
Explanation
Contrast a bond with an interest rate swap:
In a bond, the buyer pays the principal amount at issue (start), then receives coupons (computed off this principal) over the life of the bond, then receives the principal back at maturity (end).
In a swap, no principal changes hands at inception (start) or expiry (end), and in the meantime, interest payments are computed based on a notional amount, which acts as if it were the principal amount of a bond, hence the term notional principal amount, abbreviated to notional.
In simple terms the notional principal amount is essentially how much of the asset or bonds a person has. For example, if a premium bond was bought for £1 then the notional principal amount would be the face value amount of premium bond that your £1 was able to purchase. Hence the notional principal amount is the quantity of the assets and bonds.