Here's what Wikipedia says about allusion:
> An allusion is a brief and indirect reference to a person, place, thing, or idea of historical, cultural, literary, or political significance. It is a device used in literature, film, and other forms of art to create a connection between the text and something else.
Allusiveness, then, would refer to the quality of being allusive, or the tendency to make use of allusions.
Allusive writing is a style of writing that uses allusions extensively. This can be done for a variety of reasons, such as:
* To add depth and richness to the writing
* To create a sense of shared knowledge and understanding between the writer and the reader
* To evoke certain emotions or ideas
For example, Wikipedia mentions that "allusive writing is a hallmark of postmodern literature," meaning that postmodern writers often use allusions to create a sense of fragmentation and ambiguity.
While Wikipedia doesn't have a definition specifically for "allusive," it does provide valuable information on the related concepts, giving you a strong understanding of what the term implies.