Naked Single

Any cell that has only one number as a possibility is known as a Naked Single. Look at the circled cell R4C4 in the image given below.

The numbers 2,3,8,9 are eliminated as they already occur in the row 4. The numbers 4,5,6 are also eliminated since they occur in Column 4 and the number 7 is eliminated as it already occurs in Box 5. Hence the only possibility for the circled cell R4C4 is the number 1. This is known as the Naked Single Technique.