An onward ticket is a proof of purchase of a pre-booked train or airline ticket, out of the country that you are about to enter.

You have an onward ticket, if you have in your possession:

• a round-trip airline ticket
  Airline ticket
  An airline ticket is a document, issued by an airline or a travel agency, to confirm that an individual has purchased a seat on a flight on an aircraft. This document is then used to obtain a boarding pass, at the airport...
  
  
• an airline or train ticket from the country in question to another country
• a one way airline ticket to the country with the onward ticket requirement, yet only as a stop-over

When required

An onward ticket can be required, based on the countries' entry requirements (which may or may not include the onward ticket). Many (usually poorer) countries insist that you hold a flight ticket out from their country, which must be presented upon arrival at immigration. They set this requirement, so that if travellers run out of money, the country is still certain that the traveller will leave (and not stick around). In the past, it has already come forward that travellers just take a (illegal) job and remain within the country. If you do not have an onward ticket, you may be turned away in these countries with little choice, but to get back on the next plane home.

However, there are also a great number of (usually richer) countries, that do not have this ticket as an entry requirement or are less strict in the application of the law of the countries' customs. To determine which do and do not, guide book

Guide book

A guide book is a book for tourists or travelers that provides details about a geographic location, tourist destination, or itinerary. It is the written equivalent of a tour guide...

s, websites of the own Ministry of Foreign Affairs

Foreign minister

A Minister of Foreign Affairs, or foreign minister, is a cabinet minister who helps form the foreign policy of a sovereign state. The foreign minister is often regarded as the most senior ministerial position below that of the head of government . It is often granted to the deputy prime minister in...

 or the relevant embassies

Diplomatic mission

A diplomatic mission is a group of people from one state or an international inter-governmental organisation present in another state to represent the sending state/organisation in the receiving state...

and/or other sources may be used.