Loan to Value – a credit slang term often used in mortgage finance. The LTV is a ratio - the amount of a mortgage expressed as a percentage of the value of the property's value, or what you pay for the property. So if your get a mortgage with a max ltv of 90% and your property is worth £100,000 then the most you will be allowed to borrow is £90,000.
The higher the LTV, the more costly the mortgage, generally speaking. Consequently this can often tie in closely with affordability as this can restrict the maximum ltv available and therefore whether you can afford the new property.