An informal meeting between leaders, officials, or diplomats at a public event. Also: pull aside.
"The speeches from the podium are only one aspect of the diplomatic convocation, which will comprise thousands of bilateral meetings, regional conferences and focused but informal 'pull-asides' for leaders to exchange views on specific subjects."
Betsy Pisik, "Campaign against terrorism to dominate U.N. summit," The Washington Times, November 9, 2001
Q In addition to the bilateral meeting with Chinese President Jiang in Seattle, is the president going to have bilateral meetings with each and every leader of the members or just some of them?...
SR. ADMIN. OFFICIAL: Yeah. I do not think he's going to have bilateral meetings with all of the leaders. ... They are going to take off on Saturday morning and leave most of the rest of us behind and have some time for private, informal consultations. During that time, I am certain he will have the opportunity for what we call 'pull asides' with all of the leaders, a chance to talk with them one on one.
—"USIA Foreign Press Center; Background Briefing; Topic: Foreign Policy Issues of Interest to the White House," Federal News Service, October 8, 1993
The verb-to-noun background of this phrase was pithily put forward by William Safire a few years ago:
"A similar construction goes from the verb 'to pull aside,' to the diplomatic noun, a pull-aside, which means 'a private meeting at a public event.'
William Safire, "On Language," The New York Times, August 28, 1994