The word default, as used in computing, originally had the same meaning as the word used in law, and is still often used this way, especially the verb form. This fact can be used to give a definitive answer to your question.
Default: primary meaning
In both settings, default (n.) means a fault or failure, generally a failure of inaction, and default (v.) means to fail in such a way.¹ Thus, in law, to default (such as on a loan) means to fail to act properly (by not paying on the loan).² But equally, in computing, the word is used when the operator fails to take action that is necessary for the computation. Thus, in computing, to default (such as on a required input) means to fail to act properly (by not providing the required value).³
Default: computing-specific idiom, and a timeline
In computing, when the operator defaults, the programmer might choose to detect the failure and recover by continuing the computation, supplying a preprogrammed input. Because this input is only used when the operator defaults, it has come to be called the default input.
- Mid 1960s – Noun phrases such as default input, default value, and default parameter appeared frequently in print for the first time. They mean the preprogrammed value used when the operator fails (defaults) on supplying a value.
- Mid 1970s – The single word default (n.) appeared frequently in print, an idiomatic shortening that means the same thing as default value.
- Late 1970s – The set phrase take the default (to default knowing the preprogrammed default value will be supplied) first appeared in print.
(The search tool I used to develop this timeline is the Google Ngram Viewer.)⁴
So now, in computing, we still have the original term default, especially the verb form, and we also have this new term, the idiomatic shortening of default value to simply default.
From this we get the correct answer to your question. You asked whether there are any single-word alternatives to the idiomatic term default. The answer is no, there are none which also carry the original connotation of a failure to provide a value. The history of this term is unique.
JEL’s answer, preset, is closest to what you are searching for. It has no connotation of a failure to provide a value, but it has gone through the same evolutionary process. As with default, noun phrases such as preset value or preset magnitude have been idiomatically shortened to the single word preset.
There are, as other answers point out, various noun phrases such as initial value or pre-selected value, but none of these have been shortened idiomatically to a single word.