The concept "tapa" alludes more to the size of the portion than to the dish whose small portion is served in said tapa.
Complicated? Well, we'll make it easier for you: you can have a tapa of fish a la marinera, for example; or a tapa of pisto manchego; or a tapa of ensaladilla rusa. It is not a specific meal but a small portion -a tasting portion- to accompany your drink.
Moreover, beyond that, we would dare to say that tapas are a 100% Spanish sociocultural (and, as cultural, of course, gastronomic) trait, since part of the daily life of the Spanish is the so-called tapas culture, that is, "going down to the bar" -as they would say in Spain- and going from one to another drinking something -usually a wine, a mixed drink (such as sangria), beer or even a vermouth- that comes with a small portion of food.
Moreover, there are places in Spain -very typical, for example in the beautiful city of Granada- where the aforementioned drinks (wine, beer, mixed drinks or vermouth) are always accompanied by "a first", "a second", "a third"... In other words, "first, second, third..." are complimentary tapas that the bars give to those who sit down to drink something in their bars and terraces.