What is meant by Least Count (LC) of a measuring instrument?

Hint: If you see your measuring scale, you will remember that it has scaling on its both sides of different units. One side it will be in inches and the other side it will be in centimeters. You can’t measure a length in centimeters from inches side because an inch scale will measure in inches precisely.

Complete answer:
In measurement, the least count of a measuring instrument is the smallest value which can be accurately measured and resolved on its scale. The least count is related to the precision of an instrument; an instrument will be said to be more precise and having a smaller least count value, if it can measure smaller changes in a value relative to another instrument. Any measurement made by the instrument can be considered repeatable to no less than the resolution of the least count.
For example, a conventional watch may only have scales marking the hours of a day; it would have a least count of one hour whereas a stopwatch used in a race might have its least count to a hundredth of a second. So, we can say that the stopwatch is more precise at measuring time intervals than the conventional watch because it has more counts in each hour.

Note:
The least count will be inversely proportional to the precision of a measuring instrument. Least count of a measuring instrument is one of the very important tools in order to get accurate readings of instruments.