An indefinite article is a form with a certain set of functions. These functions are in a number of respects different from those of words for the number 1. Linguists look at samples of language, determine what forms exist, and determine what set of functions they fulfill. For convenience these forms will be referred to by a suitable name, based on the analysis.
Beyond this it gets a bit complicated, but here's some parts of it, focussing on properties of indefinite articles that distinguish them from the numeral 'one'. In many cases the analysis of indefinite articles will involve looking at distributional properties: where in the noun phrase (NP) does it occur, and what other elements can it co-occur with. Articles normally don't occur alone, and act as modifiers of a NP. Indefinite articles may indicate number (eg singular/plural). Semantically, articles indicate whether an item of information is a specific individual, or if it's known to the hearer or not.