In United Nations terms,

(a) a standing body, usually established by the General Assembly, devoted to the study of a particular topic, with a permanent secretariat unit linked to it, e.g. the Conference on Disarmament, the United Nations Conference on Trade and Development,

(b) a formal time-limited gathering - often called a special conference - of Governments and observers established by the General Assembly (A/CONF) or the Economic and Social Council (E/CONF) to examine a particular topic and to report thereon, e.g. the International Conference on Population and Development, the United Nations Conference on Environment and Development,

(c) a session, usually annual or biennial, of States parties to a Convention, e.g. the Conference of the Parties to the United Nations Framework Convention on Climate Change or the United Nations Convention to Combat Desertification in Those Countries Experiencing Serious Drought and/or Desertification, Particularly in Africa.

A meeting is a gathering of two or more persons for three or fewer hours in sequence (one of a series of meetings during a session) during which a legislative, administrative or deliberative body occupies a conference room and/or a space on a meeting platform and requires some conference services. 

UN Geneva provides support only for a limited number of selected mandated sessions outside the UN Geneva premises.