Legally, a conflict of interest is a conflict between a person’s personally interests and their professional obligations. Generally, the phrase is used when it seems a person’s personal interests interfere with an impersonal matter.
The generally accepted plural is conflicts of interest; however, since the word is describing the situation and not the two interests involved it does not have a formal plural. The term is always used in the singular. Two or more persons would all have a single conflict of interest. Multiple personal conflicts would still be grouped under the single conflict of interest.
Interestingly, a century ago the term conflict of interests was the most popular form of the phrase, as shown by the ngram below.
Examples
Typically, investment banks consult with their existing clients before taking on other mandates with companies in the same sector to ensure there is not a conflict of interest. [Telegraph]
House Republicans are calling on the Department of Justice to appoint a special counsel for its probe into the IRS targeting scandal, claiming new evidence shows the investigation is “compromised” due to multiple instances of conflict of interest. [FOX]
Kershaw’s manager, Don Mattingly, recently endorsed the lefty’s NL MVP Award candidacy. Granted, Mattingly has a conflict of interest here. [MLB]
Since 2008, the F.D.A. has worked to reduce the number of committee members with financial conflicts of interest; the Pham-Kanter study indicates that this effort has met with significant success. Other conflicts of interest, like professional or ideological ones, can also influence our behavior, but these have not been as well studied. And the F.D.A. is not the only place where financial conflicts of interest are a concern. [New York Times]