I like words, and I like the way in which words develop, both in form and in meaning.
However, the words from which the English language is constructed can vary in meaning, and in shades of meaning, dependent upon the form of the English language being used.
David has given us an interpretation of the word "talisman" that I am sure his quoted reference will support.
Michael has approached the subject from a slightly different direction, and has provided a slightly different interpretation that I am once again certain his references could support.
Since we are involved in discussion here in an international forum, where for most of those of us who are native English speakers, it is certain that we do not use Standard English as our everyday vehicle of communication, and for those of us who are not native English speakers, the English that we use is in the category of English as a second language, I suggest that perhaps we should use as our reference the one inarguable authority on the words which comprise the English language, and that authority is the Oxford Dictionary.
My personal favourite of the many editions of Oxford is the Oxford on Historical Principles.
If I consult Oxford H.P. in respect of "talisman", I find that development is from late Greek :- telos>telein>telesma >> Arab tilasm ( also tilsam).
telos = result, end; telein = complete, performance; telesma = completion, performance, religious rite, consecrated object.
The first recorded use of the word "talisman" in the English Language appears to have been in 1638.
Used in a formal fashion the word talisman has the meaning:-
" a stone, ring, or other object engraven with figures or characters, to which are attributed the occult powers of the planetary influences and celestial configurations under which it was made; usually worn as an amulet to avert evil from or bring fortune to the wearer; also used medicinally to impart healing virtue; hence, any object held to be endowed with magic virtue; a charm."
When the word "talisman" is used in a figurative way it means:-
"anything that acts as a charm or by which extraordinary results are achieved"
; this usage dates from 1784.
From this it can be seen that when we refer to a keris sajen as a talisman, we are referring to the virtue which we believe to be inherrent in the keris. We are not referring to the original purpose for which this keris was concieved.
In fact, even this "original purpose" could be subject to debate, for, even though the accepted name for these small keris with an integrally forged hilt, is "keris sajen", it is open to question whether all keris of this type were originally fabricated for use as a part of an offering.
I have seen it mentioned that in colonial days people in Java carried small keris as amulets, see van Duuren
Additionally a number of this type of keris that I have in my own collection I believe to have been made specifically for talismanic purposes, rather than offering purposes. The reason I believe this is because some of the very old examples have been provided with suspension rings, and some of the more recent --- perhaps 18th-19th century --- ones have been made in a non-typical form and with considerable attention to artistic detail.
Further, although we read in several sources --- which could all originate from the same root --- that these keris are offering keris, and that they are used principally in the ceremony of Bersih Desa, the two references I have that set forth the ritual for Bersih Desa make no mention of a keris of any type having any part in the ceremony. In addition to this printed evidence I have been advised by a person who was responsible for the organisation of ceremonies such as Bersih Desa that no keris is used in Bersih Desa in his village.
Possibly these keris sajen were used in some places as a part of the ceremony, and not used in other places. Or possibly what we have is a misnomer. I have no firm opinion in this matter.
We can play the semantics game till the cows come home, if we wish, and I'm probably as partial as anyone to this game, but when we get right down to tin tacks there can be no argument about the fact that the type of keris that we know as "keris sajen" is considered to be a talismanic keris by the Javanese people. Since this object is a part of Javanese culture, the important thing here is how the members of that culture regard it, not how members of different cultures might like to regard it.
For those who have not yet seen this, here are few keris sajen:-