Nowadays, the term generally is used to refer to the present participle in English when it is functioning as a noun and the verbal present participle of Spanish. These are the verb forms that end in "-ing" in English and -ando or -iendo in Spanish. Note that in English the "-ing" form isn't considered a gerund unless it's functioning as a noun, but in Spanish the gerund functions as a verb (although it can also have some of the qualities of an adverb).
Some grammarians also use the term "gerund" more loosely to refer to any verb form when it is functioning as a noun. Generally, you should not assume this definition is meant unless the context makes clear this is what is intended.