www.alphadictionary.com

Mobility, motility? One of them has to go. How did it happen that two words with the same meaning and only one letter off both make it into English? I am certain there are other examples knowing the voracious appetite of our language.

They don't mean the same thing. Both refer to movement, but one, mobility, is the possibility of being moved, whereas the other, motility, is the capability of self-movement.

Lots of people may have mobile phones and love them, but I'm sure they would not appreciate motile ones. They'd never know where they were.

Mostly I have heard motility used in reference to body processes or parts.