Nowadays, the term generally is used to refer to the present participle in English and the verbal present participle of Spanish. These are the verb forms that end in "-ing" in English and -ando or -iendo in Spanish. In both languages, the gerund is used to form the progressive or continuous tenses. Note that while the English gerund can be and frequently is used as a noun, the Spanish gerund does not function as a noun.
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.