Nearly all nouns in Spanish are always masculine or always feminine. But there are a few nouns that can be either.

In most cases, those are the nouns describing what people do, and the gender varies with the person the word stands for. Thus, for example, el dentista refers to a male dentist, while la dentista refers to a female dentist.

But there are a few nouns where the matter of gender is more complicated. Those are the nouns whose meanings vary depending on the gender of articles or adjectives used with them. In most cases, there's little logical reason why one meaning has developed with the masculine gender and another meaning for the feminine, so the only ways to learn them is to memorize them or use them until you know them.

Here's a list of the most common such words. Only the basic or most usual meanings are included here; consult a dictionary for more thorough definitions.

Footnotes:

1. Some authorities indicate that escucha is feminine in all uses.
2. In some areas, radio is masculine in all uses.