According to the American Heritage Dictionary, a slant rhyme is "A partial or imperfect rhyme, often using assonance or consonance only." Slant rhyme is also known as half rhyme or off rhyme. In a slant rhyme, the last word on each line does not exactly rhyme with the line before it, but it sounds close enough to flow together. Not all of the vowel and consonant sounds rhyme like they would in a perfect rhyme. An example of slant rhyme are the words "hurtful" and "purple." If you try those words in a rhyme together, you will see that they do not rhyme in the true sense, but they still flow well together.
Slant rhyme was first used by Irish, Welsh and Icelandic poets. It did not become popular in English poetry until poets like Yeats came along and used it successfully. Emily Dickinson also used slant rhyme in most of her poetry, and her poetry made the style more mainstream. Slant rhyme was difficult to accept back then because most of the poetry was just written word, and no one heard it performed. When rap music came onto the scene, it was easy to sell listeners on slant rhyme because of the way the rapper made the words flow with his or her pronunciation and careful emphasis on certain sounds.
Slant rhyme can be used when you need to use a word in your poem or rap, but that word has no other words that rhyme perfectly with it. This will prevent you to from having to change words you really want to use just because they do not rhyme. Some examples of words for which no perfect rhymes exist are "month" and "wolf." Rappers also use slant rhyme to avoid using rhymes that have been used over and over again. Rhymes like "honey and money" have been used millions of times in poems and raps. Instead, a poet or rapper could use slant rhyme with the words "money and from me."
The key to making slant rhyme work in a song or poem is how you pronounce and emphasize the words you are slanting. In a performance or a reading, the audience may not even notice that your words do not rhyme if you make them sound alike with your pronunciation. For example, if you are slanting the words knowledge and polish, you would emphasize the "ah" sound in each word to trick the listeners ears and make them follow the flow of your rhymes.