Definition of ECI

ECI = Early Career Investigator, which implies that researcher's career spans less than 8 years since the PhD.

ECI is a replacement of the term ESR (early stage researcher). The ESR was in use until January 2015, when it was superseded by the term ECI (early career investigator). Here is the COST definition of the two terms:

Early Stage Researcher - was a term used until 1 January 2015 to define a researcher whose career spun less than 8 years between the date of the PhD/doctorate (or similar experience) and the date of involvement in the COST Action. Periods of career leave have to be added to this time span. According to the COST Rules valid as of 1 January 2015, the corresponding term refers to Early Career Investigators (ECI).