SOP stands for standard operating procedure. That is the short answer and it is rarely the useful one, because the question is usually asked by somebody who has been told to write one and does not know how it differs from the three documents around it. Policy, procedure, work instruction and checklist are four different things, and being asked for the wrong one wastes a week. This page names all four.
Policy: the rule, signed off by somebody senior
What the organisation requires and why, in a paragraph or two. It changes rarely, it names no systems and no individuals, and it is approved by whoever owns the risk. If what you are being asked for could be settled by a board decision, you are being asked for a policy and it should be short.
Procedure, the SOP itself: how a role satisfies the rule
The steps one role follows to do one job, in order, with the checks and what to do on exception. It names systems, it names roles rather than people, and it changes whenever those change. This is the layer where a register lives: the worked example on this site is 48 of them.
Work instruction: the keystrokes, and usually unnecessary
A step-by-step at the level of which button to press. Genuinely needed where the task is unfamiliar or the consequence of error is severe, and a waste of maintenance everywhere else, because it is the layer that goes out of date fastest. Most organisations that adopt three layers do it because a template had three.
Checklist: the thing people actually use
The short list somebody works through while doing the job, derived from the procedure. It is not a substitute for the procedure, because it carries none of the reasoning, and it is frequently the only artefact that gets used. A register that produces both from one source is doing the useful thing.
Questions people ask about sop stands for
What does SOP stand for?
Standard operating procedure: the steps one role follows to do one job, with checks and exceptions.
How is an SOP different from a work instruction?
A work instruction is at the level of keystrokes. It is worth writing only where the task is unfamiliar or an error is severe, because it ages fastest.
Do we need all four document types?
Most organisations need two: policy and procedure. Three-layer structures are usually adopted from a template rather than needed.