"(A) The words or the language, as they are written or spoken, do not seem to play any role in my mechanism of thought. The psychical entities [I emboldened.] which seem to serve as elements in thought are certain signs and more or less clear images which can be "voluntarily" reproduced and combined. There is, of course, a certain connection between those elements and relevant logical concepts. It is also clear that the desire to arrive finally at logically connected concepts is the emotional basis of this rather vague play with the above-mentioned elements. But taken from a psychological viewpoint, this combinatory play seems to be the essential feature in productive thought--before there is any connection with logical construction in words or other kinds of signs which can be communicated to others.
From "A Mathematician's Mind, Testimonial for An Essay on the Psychology of Invention in the Mathematical Field by Jacques S. Hadamard, Princeton University Press, 1945." in Ideas and Opinions.
Einstein means : mental images.
A.Einstein answered to a questionnaire issued by the French mathematician Jacques Hadamard regarding "THE WORKING METHODS OF MATHEMATICIANS", originally issued in L'Enseignement Mathematique (1902-1904).
Einstein letter is reproduced into : Jacques Hadamard, The Psychology of Invention in the Mathematical Field, original edition: 1949, page 142.
The quote answer to Hadamard's question n°.30 [see footnote 1, page 142] :
[page 140] 30. It would be very helpful for the purpose of psychological investigation to know what internal or mental images [emphasis added], what kind of "internal word" mathematicians make use of; whether they are motor, auditory, visual, or mixed, depending on the subject which they are studying.