Sensitization – “exposure to allergen that results in the development of hypersensitivity.” 
So what is hypersensitivity? Is a hypersensitivity reaction the same as an allergic reaction?
Answer, yes. They are synonyms, BUT there are four different types of allergic reaction:
“a local or general reaction of an organism following contact with a specific allergen to which it has been previously exposed and sensitized; immunologic mechanisms gives rise to inflammation or tissue damage. Allergic reactions are classified into four major types: type I, anaphylactic and IgE dependent; type II, cytotoxic; type III, immune-complex mediated;type IV, cell mediated (delayed).”
For the purposes of aromatherapy safety, any essential oil can become an allergen by using it undiluted on the skin; and this risk is there for all essential oils, including lavender. So while there are certain essential oils which have a known reputation for being potential allergens or with a reputation for sensitization, using any essential oil neat (undiluted) sets the individul up for a potential allergic reaction, leading to sensitization, and forever being allergic to that essential oil.
Robert Tisserand explains:
“Yes, sensitization is the process that takes place in the body that leads to an allergic reaction. They are not the same thing, but they are not totally different either. There are 4 types of allergic reaction,  but only two are relevant to essential oils. Type 4 (delayed hypersensitivity) accounts for 90% of allergic reactions. Type 1 is immediate hypersensitivity (generally not anaphylactic) and accounts for the other 10%. You could say the risk is potentially there for all essential oils, but this is a little unfair on the majority of oils, that have never been known to cause such reactions. I don’t like to assume risk that may not exist. The less you dilute the more you increase risk, but that doesn’t mean that undiluted copaiba oil is a greater risk than 1% cinnamon bark oil. It isn’t.”
To learn more, see the video which is a sample from Webinar 4 “Irritation and Allergy” from Essential Oils and the Skin 10-Day Series by the Tisserand Institute: https://www.facebook.com/video.php?v=898998096798059
1. Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved
2. Farlex Partner Medical Dictionary © Farlex 2012