Rallentando and Ritardando

[Targa]

My dictionary defines rallentando as 'becoming slower' and ritardando as 'with diminishing speed'.
What's the musical difference or are they synonyms.

 

[Young Col]

Targa
The Royal Schools of Music guide has them as synonymous - gradually getting slower.

 

[Col9]

We play them as "rit" which is instantly slower where marked like a step, and "ral" which starts to slow where marked like a gradient.

Tricky to get it right while reading the music in one eye and watching conductor in the other ;}

Col.