Of course, some words, such as names of many occupations, are masculine when they refer to men and feminine when they refer to women, as in el dentista for male dentist and la dentista for female dentist. And there are some nouns whose meanings vary with gender. However, there are also a number of words that, for whatever reason, haven't been firmly established as being of one gender or the other.
Following is a list of the most common of these words. Where just el or la appears before the word, it is the gender that is viewed most widely as correct, and the gender that should be learned by foreigners. Where both appear, either gender is widely accepted, although the most commonly used gender is listed first. Where no gender is listed, the usage depends on region.
la acné — acne
el anatema — anathema
el arte — art — The masculine is used when arte is singular, but the feminine is often used in the plural, as in artes bellas (fine arts).
el autoclave — sterilizer
el azúcar — sugar — Although azúcar is a masculine word when standing alone, it is often used with feminine adjectives, as in azúcar blanca (white sugar).
la babel — bedlam
el calor — heat — The feminine form is archaic.
la/el chinche — small insect
el cochambre — dirt
el color — color — The feminine form is archaic.
el cutis — complexion
la dote — talent
la duermevela — brief, light or interrupted sleep — Compound nouns formed by joining a third-person verb and a noun are nearly always masculine. However, the ending apparently has influenced usage of this word toward the feminine.
el enema — enema
los herpes — herpes
la/el Internet — Internet — The general rule is that nouns imported from other languages are masculine unless there's a reason for making them feminine. In this case, the feminine is often used because the word for a computer network (red) is feminine.
el interrogante — question
la Janucá — Hanukkah — Unlike the names of most holidays, Janucá is usually used without a definite article.
el/la lente, los/las lentes — lens, glasses
la libido — libido — Some authorities say that libido and mano (hand) are the only Spanish nouns ending in -o, other than shortened forms of longer words (such as foto for fotografía and disco for discoteca, or occupational words, such as la piloto for a female pilot), that are feminine. However, the word is often treated as masculine.
la/el linde — boundary
el mar — sea — Mar is usually masculine, but it becomes feminine in some weather and nautical usages (such as en alta mar, on the high seas).
el/la mimbre — willow
la/el pelambre — thick hair
el/la prez — esteem, honor
la/el pringue — grease
radio — radio — When it means "radius" or "radium," radio is invariably masculine. When it means "radio," it is feminine in some areas (such as Spain), masculine in others (such as Mexico).
el reuma — rheumatism
sartén — frying pan — The word is masculine in Spain, feminine in much of Latin America.
la testuz — forehead of an animal
la tilde — tilde, accent mark
el tizne — soot, stain
el tortícolis — stiff neck
la treponema — type of bacteria — Like some other words of limited medical usage, this word is feminine according to dictionaries, but usually masculine in actual use.
el trípode — tripod
la/el web — web page, web site, World Wide Web — This word may have entered the language as a shorter form of la página web (web page), or it may be feminine because red (another word for the Web) is feminine.
el yoga — yoga — Dictionaries list the word as masculine, but the ending has led to some feminine usage.