The French word frontière is derived from the military term front, referring to the contact zone with an enemy force. It is a line that fluctuates according to the advances and retreats of the forces facing one another. In English this gives the word "frontier", which has overall retained this meaning (although "border" coexists, appearing as early as Chaucer to mean the marches or frontier zone). However the English "frontier" also carries the marked cultural connotation of the western "wild" frontier in the USA. From the 17th century, the French frontière gradually took on the meaning of a fixed boundary, or border between two states, and is the term used today for territorial borders or boundaries, in particular international borders.
Frontière, like border, took on a more political meaning with the appearance of nation-states, and from then onwards came to be based on the notion of a natural boundary: the boundary of a territory is all the easier to apprehend and control when it is based on a physical boundary feature. Frontière or border thence became a line, the contours of which were sometime artificial, and which could be the subject of negotiation. Its existence is however legitimated by the concept of a natural boundary. Thus the political border or frontière consists in a separating boundary or dividing line between two territories (two exercising sovereignties), materialised by the existence of a discontinuity often represented by a line. Two political systems are thus face to face, and function on equal footing, but their modes of functioning and organisation and their legal systems differ.
The border/frontière then entails a powerful system of control that is more or less explicit (system of defence, customs control etc) the main object of which is to protect, but also to allow circulation while at the same time filtering it and drawing advantage from it. Certain exchanges are actually generated by the presence of a border (on account of differentials) while other exchanges use illegal channels to escape control. Alongside the soldier and the customs officer, the smuggler is the third emblematic figure of border areas, the first two protecting and filtering, and the third embodying passage and in-betweenness. This ambivalence between separateness and exchange is characteristic of the border. A border functions as an unstable filter, with alternation of phases of openness (contact prevails over separation) and phases of closure (separation prevails over contact) to varying degrees: the porosity of a border varies over time, and according to relationships between the systems on either side.
In other fields of social and human sciences, the notion of the "frontier" or "boundary" (rather than "border") and the French "frontière" are used in a wider meaning not specifically referring to state boundaries. These terms can be used to refer to divisions between cultural groups (e.g. religious or linguistic boundary) and this involves the notion of imprecise, fluctuating contact. The English use the word "boundary" for the limits of a sports field, or the expression "out of bounds" can also be noted in this respect.
Thus there are two notions behind the word frontière in French, one is in line with frontier and boundary in English and entails a cultural and social dimension, the other has a more political dimension corresponding to the English border and also boundary. The semantic overlap is complex.
These different dimensions are an invitation to widen the scope of the classic definition of frontière in geography: a "frontière" (for the sake of argument, boundary) is a geographical object separating two contiguous territorial systems. This object cannot be reduced to a dividing line, since it has an effect on the organisation of space (border effects) and also because it integrates a political dimension (i.e. what affects the structuring of society), a symbolic dimension (it is recognised by all protagonists and serves as a marker in space) and a material dimension (as part of the landscape).
A boundary/border/frontière exists when territorial systems identified by their own sets of norms (cultural, legal, etc) confront one another. This is indeed the case for nation states, with their institutional functioning, their networks etc. But the definition can also be extended to include other emerging territorial systems. This opens up new lines of research in the context of a crisis of the State, and the emergence of new powers (economic, institutional or social in nature). It can therefore be proposed that boundaries/borders (frontières) appear in forms other than a line, as a point (a port or airport), a zone, or a front. The classic representation of the border (frontière) is then overturned. A fresh line of research would be to suggest new modes of representation for these novel forms of boundary.
See also: “discontinuity”