What is the meaning of indentation?

noun

• A concave cut into a surface or edge (as in a coastline.
  • synonyms: indenture
• The formation of small pits in a surface as a consequence of corrosion.
  • synonyms: pitting, roughness
• The space left between the margin and the start of an indented line.
  • synonyms: indention, indent, indenture
• The act of cutting into an edge with toothlike notches or angular incisions. .