step·ping·stone 
(stĕpĭng-stōn′)
(stĕpĭng-stōn′)n.
1. A stone that provides a place to step, as in crossing a stream.
2. An advantageous position for advancement toward a goal.
- photodiode
- photodisintegration
- photoduplicate
- photoduplication
- photodynamic
- photodynamically
- photodynamics
- photodynamic therapy
- photoelectric
- photoelectrical
- photoelectrically
- photoelectric cell
- photoelectric effect
- photoelectron
- photoemission
- photoemission spectroscopy
- photoengrave
- photoengraver
- photoengraving
- photo-essay
- photo essay
- photo-essayist
- photo finish
- photoflash
- photoflood