1. The European Commission favours nowadays the term ‘irregular entry’ (e.g. see: DG Home e-Library Glossary )
2. Art. 5 of the Regulation (EU) 2016/399 (Schengen Borders Code) states that for stays not exceeding three months per six-month period, the entry conditions for third-country nationals shall be the following:
a) they are in possession of a valid travel document or documents authorising them to cross the border;
b) they are in possession of a valid visa , if required pursuant to Council Regulation (EC) No 539/2001 listing the third countries whose nationals must be in possession of visas when crossing the external borders and those whose nationals are exempt from that requirement (17), except where they hold a valid residence permit;
c) they justify the purpose and conditions of the intended stay, and they have sufficient means of subsistence, both for the duration of the intended stay and for the return to their country of origin or transit to a third country into which they are certain to be admitted, or are in a position to acquire such means lawfully;
d) they are not persons for whom an alert has been issued in the SIS for the purposes of refusing entry;
e) they are not considered to be a threat to public policy, internal security, public health or the international relations of any of the EU Member States, in particular where no alert has been issued in EU Member States' national data bases for the purposes of refusing entry on the same grounds.
3. For more information, see Regulation (EU) 2016/399 (Schengen Borders Code) .