Sorry for this english related question but I only came across that expression in the context of IT. What does abstracting over something mean ? For example abstracting over objects or abstracting over classes. Thanks
In this context, the word "abstract" comes from the lambda calculus, where it means "to make something a parameter" (a value parameter or a type parameter). The word is used more generally with other kinds of parameters; for example, mechanisms for "generic programming" often include ways of abstracting over classes.
Probably the easiest language in which to abstract over objects and classes is Smalltalk, where everything (including every class) is an object. Smalltalk, like Ruby which is closely based on Smalltalk, has "duck typing", so for example you could "abstract over" any collection class by writing Smalltalk code that uses only methods common to all collection classes. You could abstract over collection objects in a similar way.