Although not wholly adequate, Matthew Fox's definition of prayer is still my favorite. It is a definition that doesn't even use the word 'God'. Fox defines it as "a radical response to the mysteries of life."
Three things make this definition so meaningful to me. The first is the word 'radical', which comes from the Latin 'radix', for "root." It implies that prayer requires that we get at the root of things without being distracted by superficialities. We must think deeply about our lives, which, of course, takes time. It is a contemplative definition.
The second is the word response. It implies that through prayer we must not only think deeply but also translate our thinking into action. We need to behave out of deep thoughtfulness.
Finally, it implies that life is an inherently mysterious business and that there are no easy answers or simple formulas. Even those answers we arrive at through our best thinking will not necessarily be crystal-clear ones that relieve us from the burden of taking risks.
--M. Scott Peck