What does R stand for in S-M-A-R-T rule?

SMART

is a mnemonic acronym, giving criteria to guide in the setting of objectives, for example in project management, employee-performance management and personal development. The letters S and M usually mean specific and measurable. The other letters have meant different things to different authors, as described below. Additional letters have been added by some authors.

SMART criteria are commonly attributed to Peter Drucker's management by objectives concept. The first-known use of the term occurs in the November issue of Management Review by George T. Doran. The principal advantage of SMART objectives is that they are easier to understand and to know when they have been done.

The November issue of Management Review contained a paper by George T. Doran called There's a S.M.A.R.T. way to write management's goals and objectives. It discussed the importance of objectives and the difficulty of setting them.

Ideally speaking, each corporate, department, and section objective should be:

• Specific – target a specific area for improvement.
• Measurable – quantify or at least suggest an indicator of progress.
• Attainable – assuring that an end can be achieved.
• Relevant – is the right goal at the right time for you.
• Time-related – specify when the result(s) can be achieved.

Notice that these criteria don’t say that all objectives must be quantified on all levels of management. In certain situations it is not realistic to attempt quantification, particularly in staff middle-management positions. Practising managers and corporations can lose the benefit of a more abstract objective in order to gain quantification. It is the combination of the objective and its action plan that is really important. Therefore serious management should focus on these twins and not just the objective.

Agreed and learned from Mr. Ihab El Mortada's answer. Thanks.

realistic  i.e .the content should be related to the topic

You should try to use google before asking questions !! sorry

R stands for Realistic. As mentioned above.

R stands for Relevant. A goal set needs to be relevant to the persons role

In SMART Rule: R Stand For Realistic, achievable attainable

R- Reliable and responsible

R stands for: realistic and resourced