Expression

Expression is an impossible word.
If you want to use it
I think you have to
explain it further…
— Ad Reinhardt,
Art as Art

“… an equation is a very
abstract expression
of knowledge
about something.”
— The Hidden Side
of Visualization

Well, yes.
For example:

The image “http://www.log24.com/log/pix05B/051230-Shema.jpg” cannot be displayed, because it contains errors.
Abstract Expression,
pixels on screen, 12/30/05

The Scope and Limits
of Quotation
(pdf), in
The Philosophy of
Donald Davidson,
ed. L. E. Hahn,
Open Court Publishers,
1999, pp. 691-714,
by Dr. Ernest Lepore,
Rutgers University:

“… an assumption I wish to reject, namely, that in quotation an abstract expression (shape) is denoted….
Since Davidson, however, only says ‘we may  take [an expression] to be an abstract shape’ [1979, p.85, my emphasis], his theory is compatible with expressions being something else. We need only find something that can be instantiated by radically differently shaped objects. Whatever it is must be such that written tokens, spoken tokens, signed tokens, Braille tokens, Semaphore tokens, finger language tokens, and any other way in which words can be produced, can be instantiated by it….
Such entities might  exist; if they do, they might ultimately play some role in the metaphysics of language.”
My emphasis.
Reference:
Davidson, D., 1979, “Quotation,” in Inquiries into Truth and Interpretation , Oxford, Oxford University Press, 1984, pp.79-92.

For more on zero and other entities,
see Is Nothing Sacred?

For more on Davidson,
see Shema .