How to present an abstract class in UML class diagram

You have to write the class name in italic :

Here Contexte is an abstract class and Structure is a “concrete” class

P. 99 of the specs says:

The name of an abstract Classifier is shown in italics, where permitted by the font in use. Alternatively or in addition, an abstract Classifier may be shown using the textual annotation {abstract} after or below its name.