Abbreviation for "year to date". Usually means the period starting 1 January of the current year and ending today.
Yield to maturity is the rate of return expected if a bond or other dated investment is held for the full term of the contract, or until the maturity date.
Rate of return on an investment in percentage terms, taking into account annual income and any change in capital value. Also the dividend payable on a share expressed as a percentage of the market price.