Data imputation simply means replacing the missing data with something.

There are multiple ways to impute data,

1. Mean
2. Median
3. Mode
4. Replace with most co-related data (see Finding Co-relation between two data or distribution)
5. Assign new category i.e. unknown (For categorical values)
6. Interpolation (Time series data)
7. Use K-nearest Neighbor (KNN) to interpolate data