C. H. Dodd's definition of a parable

The most thought provoking definition of parable comes from C. H. Dodd, The Parables of the Kingdom, Rev. ed (London: James Nisbet & Co., 1961)

“At its simplest, the parable is a metaphor or simile drawn from nature or common life, arresting the hearer by its vividness or strangeness, and leaving the mind in sufficient doubt about its precise application to tease it into active thought.”

This definition allows a parable to be either a metaphor and/or a simile (sometimes a hangup issue among biblical scholars). But the key is the fascinating possibility that the parable creates a "sufficient doubt" thus provoking thought or "teasing" out thought/reflection. What a wonderful hope for keeping Scripture from being dead dogma, and being kept alive by the Holy Spirit. One might even carefully speculate that the Holy Spirit provokes doubt or, at least, is in the midst of it!