Limerence is a state of mind sometimes referred to as "being in love" (as distinct from "loving" someone) and sometimes called "infatuation." However, the term "infatuation" carries connotations of immaturity that "limerence" separates from the emotion.
The word "limerence" was coined by Dorothy Tennov while a professor of psychology at the University of Bridgeport, Connecticut around 1977, and first published in her book Love and Limerence. "Limerent" is the subjective noun (the limerent person) as well as the adjective form (so the "limerent object" is the person the limerent desires). The coinages are arbitrary; there is no specific etymology.
"Limerence" is distinguished from "love" in that love (in most of its meanings) involves concern for the loved one's welfare and feelings with little or no expectation of gain in return. In contrast, limerence demands reciprocation.
Also in distinction from love, limerence tends to be comparatively short-lived. It can last one and half years at most, but typically lasts only few months. Either reciprocation occurs and limerence is replaced by other feelings (possibly lasting love), evidence that the limerent object does not reciprocate finally overwhelms the limerent's passion, or the limerence is transferred to a new object.
The primary characteristics of limerence can be summarized as intrusive, perhaps obsessive thinking about the limerent object and acute longing for reciprocation. Clinically, this state is marked by decrease of serotonin neurotransmitter, which leads to increase in emotional sensitivity and instability. People can become very irrational, almost insane. In particular, the desire for reciprocation can produce irrational beliefs ("she only had me arrested because her love is too strong for her to stand") and behavior ("if I cut off my ear she will see how much I love her"). Limerents often feel real physiological effects, including a physical pain in the chest ("heartache") when reciprocation seems unlikely, and euphoria when reciprocation seems evident. If love object does not reciprocate and does not handle the situation with care, people in this state can suffer severe depression, sometimes committing suicide - unrequited love is one of the major causes of suicide in the younger population.