What is Cell Padding?

Now a days I'm hearing the word of Cell Padding. What is meant by that?

Cell Padding refers to placement clearance applied to std cells in PnR tools. This is typically done to ease placement congestion or reserve some space for future use down the flow.
For example typically people apply cell padding to the buffers/inverters used to build clock tree, so that space is reserved to insert DECAP cells near them after CTS.

Just a hollow region defined around a cell for future use . Most generally defined around macros in the design for buffer placement and as f1freak said for decaps as well as tie cells .

Hi All,
I got a clear explanation for cell padding.

Cell padding adds hard constraints to placement. The constraints are honored by cell legalization, CTS, and timing optimization, unless the padding is reset after placement so
those operations can use the reserved space. You can use cell padding to reserve space for routing.

The command "specifyCellPad" is used to specify the cell padding in SOC-Encounter.

This command adds padding on the right side of library cells during placement.

The padding is specified in terms of a factor that is applied to the metal2 pitch. For example, if you specify a factor of 2, the software ensures that there is additional clearance of two times the metal2 pitch on the right side of the specified cells.