As a frequent poster on several of the major hair boards, this is a question that often comes up. What’s the difference between regular conditioner and deep conditioner? A lot of people say that it’s just the length of time that you leave the conditioner on your hair that determines if it is “deep” or not. Others say that you can add stuff “oils/butters/humectants” to a regular conditioner to get the effects of a deep conditioner. In my opinion, to actually be able to answer that question, you have to delve a bit deeper into the science of hair, specifically the structure of the actual hair.
Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair and makes up the majority of the hair shaft; and the cuticle, the hard outer layer that protects the shaft. The cuticle is formed by tightly packed scales in an overlapping structure similar to roof shingles. On hair that is in optimal condition, the scales on the cuticle lie flat and are tightly packed.
Here is a diagram that I found that best show what I'm describing
In addition to these natural oils, there are other ingredients that have been proven to penetrate the hair shaft and provide a conditioning effect namely: