The value that you are trying to find when you make such a measurement has a true value which is unknown and is fundamentally unknowable. For example, if the above line were measured with several different rulers, a series of measurements would be obtained, each of which might be slightly different. The difference between the true value and any given measured value is called the error in the measurement. Experimental error , when used in this context, has a very specific meaning and does not necessarily imply a mistake or blunder. If you know about a mistake or blunder, you can, at least in principle, fix the problem and eliminate the mistake.
Some experimental error is intrinsic.
While it can be minimized, it cannot be eliminated. A perfectly executed experiment. Determinate error is also called systematic error. Indeterminate errors arise from uncertainties in a measurement as discussed above. Indeterminate error is also called random error, or noise.