Spread

Also known as Credit Spread

This is the risk which offsets the additional risk with a higher yield.
Credit spread risk is only exhibited when a mark-to-market accounting policy is applied.
Credit spreads are the most commonly used indicator of the risk-return profile of a bond.
The spread is the difference between the yield on a long term bond and a short term bond
There are a number of different types of credit spread:

Bond Spread

Also known as Nominal Spread or Credit Spread
This is the extra yield over the government bonds risk free rate
This provides an easy way to compare two bonds with the same maturity date.

Interpolated Spread

Also known as I Spread or Swap Spread
This is the difference between the a Bonds Yield to Maturity and the Swap Rate.

Zero Volatility Spread

Using Zero Volatility Spread
The Z spread is the constant spread that makes the price of a security equal to the present value of its cash flows when added to each treasury spot rate.

Option Adjusted Spread

Using Option Adjusted Spread
If a security has embedded options we can use the "option adjusted spread".
There are 2 ways to derive the option adjusted spread:
1) binomial model - when the cash flows are not interest rate path dependent (for example, callable and puttable bonds)
2) monte carlo model - when the cash flows are interest rate path dependent (for example mortgage backed securities and home equity ABS)

Asset Swap Spread

Using Asset Swap Spread

Important

In practice traders use the asset-swap spread and the z-spread as the main measures of relative value.
When the economy is expected to grow at a fast pace, spreads tend to tighten.
When the economy is expected to shrink at a slower pace, spreads widen.