The yield to call (YTC) is the yield on callable bond, on the assumption that it will be called on the earliest possible date.
The calculation of YTC is similar to the yield to maturity: it is, in fact the same calculation, except that the final cashflow is the call price and it only includes coupon payments prior to the next call date.
There may be more than one call date, in which case there will be more than one yield to call number. These are usually distinguished by calling them yield to first call, yield to second call etc.