What is the antonym for "default" (computer science). For example there is "default value", how to say "not default"? Is there a one word for "not default" :)?
I know a bit of programming. And, it's surprising (I confirmed it with a profound programmer) that an opposite word to default value (strictly in computer science) is
Some programming languages take default values unless you define them. The moment you define them, they are overwritten.
So, I think in your context, not default is defined.
Other good alternatives, as said by Damkerng are customized, user-defined, specific.
This word has no clear semantic antonym.
It has a grammatical antonym, of course, which is simply the word non-default: the non prefix is used, usually with a hyphen. undefault is strange as an adjective, and indefault or adefault completely so. The un prefix can be used to negate the participial adjective defaulted, which is derived from the verb to default. If a financial loan is in good standing (payments are being made) we might say that it's an undefaulted loan. The question is, what does non-default mean: see the bottom of the answer.
A given parameter in a system can be either undefined, system-defined (default) or user-defined. Sometimes there is no system-defined state: a value for the parameter must be supplied, or else it is undefined.
Initial values that occur by fluke, and are not documented, are not defaults. For instance, if the value 42 occurs in RAM location 500 when power is applied to a device, isn't a default, even if it appears to be reliable on every power up. There is no documented guarantee, and the value may be different in another device from a different production run where a different memory chip was used or whatever. Defaults are predictable, documented initial values which are defined by the system (operating system, programming language, application, ...).
In some systems, parameters can begin in an undefined state, and there is an initialization procedure which sets them to default values, which can be further configured ("overridden", "customized", "tweaked", ...).
In some systems, when values such as variables are defined, they never have an uninitialized state; if no initial value is supplied, there is an implicit one, and that constitutes a default.
Sometimes there are multiple levels of defaulting. A programming language might default global variables to zero. However, a program written in the language might define some variables with non-zero values. To the language, these values are non-default values, but to the user of the program, they are documented as that program's defaults.
It is not clear whether the initial, undefined state is the "opposite" of the default configuration, or whether the overridden states are the opposite, and this becomes even less clear if there are multiple levels of defaulting.
A user who only sticks with the default values will probably regard the uninitialized state as the opposite: the opposite of the configured (default) state. ("Order versus chaos.")
A user who doesn't think about the uninitialized state, but chooses between default and customized configurations might regard those two as opposites. ("My choice versus someone else's.")
The meaning of the non-default grammatical antonym of default tends to lean toward the latter definition. It would be strange to refer to a nonexistent or uninitialized value as non-default (even though, strictly speaking, that is true); non-default usually refers to a well-defined alternative to the default.