In human chemistry, interpersonal chemistry is a term used, in a colloquial sense, to define a type of felt or experienced “chemistry” between people in friendships, associations, or relationships.  The term is an etymological evolution of the conception of “chemistry” between people or older versions of “alchemy” between people. In modern terms, the use of the phrase interpersonal chemistry is common.
In a 2007 interview of the science-based matching cite eHarmony.com’s current CEO Greg Waldorf, self-taught computer programmer and Harvard business school graduate, he stated:
“I love the challenge, that people think: how could you possibly measure interpersonal chemistry?”
He continues, "I like the idea of research and measurement around these things that seem so impossible to quantify."  In 2008, the new “eHarmony Labs”, whose motto is: dedicated to the study of human relationships, furnished complete with couches, chairs, tables, and video equipment, which looks like sets of miniature living rooms, supposedly is setting out to measure and quantify “interpersonal chemistry” between people. 
The idea of the existence of “chemistry” between people that a number of definitions exist:
Interpersonal chemistry – the way two individuals relate to each other (WordNet Princeton University, 2003).
Chemistry – a reaction between two people or the spontaneous reaction of two people to each other, especially a mutual sense of attraction or understanding (Encarta, 2006)
Chemistry – the interaction of one personality with another or sympathetic understanding (Random House Unabridged Dictionary, 2006). Chemistry – a strong mutual attraction, attachment, or sympathy (Merriam Webster, 2002)
Chemistry – the elements of a complex entity and their dynamic interrelation; mutual attraction or sympathy (American Heritage Dictionary of the English Language, 2000).
Chemistry – any complex, especially emotional, change or process; a person’s personality or temperament (Oxford Illustrated American Dictionary, 1998)
Chemistry – instinctual attraction or affinity (from the alchemical sense) (Online Etymology Dictionary, c. 1600).
Chemistry – an instinctual, apparently unanalysable, attraction or affinity between people or groups of people; the combination of personal characteristics that create this (Oxford English Dictionary, c. 1600).
1. (a) Interpersonal chemistry (definition) - WordNet 3.0, Farlex clipart collection. © 2003-2008 Princeton University, Farlex Inc.
(b) Smethurst, Steve. (2005). “It’s About Interpersonal Chemistry”, The Times, 10 Feb.
2. Parker, Emily. (2007). “The Matchmaker: Log on, Subscribe, Meet, Marry”, The Wall Street Journal Online, The Weekend Interview, 10 Feb.
3. (b) eHarmony Labs – Who knew that love and science would be so compatible?
(b) Tierney, John. (2008). “Hitting It Off, Thanks to Algorithms of Love”, New York Time, Science, 29 Jan.