A Proof which indirectly shows a mathematical object exists without providing a specific example or algorithm for producing an example.
See also Proof
Courant, R. and Robbins, H. ``The Indirect Method of Proof.'' §2.4.4 in
What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed.
Oxford, England: Oxford University Press, pp. 86-87, 1996.