Here are some useful definitions.
An estimation is an approximation of a quantity which is decided by judgement rather than by carrying out the process needed to produce a more accurate answer.
An approximation is a stated value of a number that is close to (but not equal to) the true value of that number. It is obtained by carrying out a process, which produces a more accurate answer. The process might include measuring, doing a sum or any other arithmetic procedure. Several reasonable approximations are always possible for any number. The one most suitable for the purpose in hand must be chosen.
An error is the difference between the value of an approximation and/or estimation and its true value. It may or may not have (+) or (-) signs attached.
The accuracy of a number is an indication of how exact it is. Very often in numerical work and when making measurements (except for distinct objects), the answer cannot be an exact one. Thus, it is necessary to indicate how accurate it is.
*obtained from The Oxford Mathematics Study Dictionary