Answer: The term "defective verb" (verbo defectivo) is applied to at least three different types of verbs in Spanish:
1. Verbs that logically are conjugated in the third person only. These verbs, sometimes known as impersonal verbs, are the verbs of weather and natural phenomena, such as amanecer (to dawn), anochecer (to get dark), helar (to freeze), granizar (to hail), llover (to rain), nevar (to snow), relampaguear (to flash lightning) and tronar (to thunder).
Occasionally, you may see or hear these verbs used in a personal or figurative sense in other than the third person, although such usage is quite rare. If one were, for example, anthropomorphizing Mother Nature and she were speaking in the first person, it would be more common to use an expression such as hago nieve (literally, "I make snow") rather than coining a first-person construction of nevar.
2. Verbs where certain forms of conjugation don't exist. Spanish has a handful of verbs that some authorities indicate don't exist in all conjugations, although there is no apparent logical reason why they wouldn't. The most common of these is abolir ("to abolish"), which some grammar guides and dictionaries say is conjugated only in forms where the suffix begins with -i. (The illegitimate forms include most present-tense conjugations and some commands.) Thus, for example, according to these authorities, abolimos ("we abolish") is a legitimate conjugation, but abolo ("I abolish") is not.
Despite this "rule," however, supposedly unacceptable conjugations are used in real life. A recent news story appearing in Spanish-language publications, for example, stated that "el presidente ucraniano ha promulgado una ley que abole la pena de muerte" ("the Ukranian president has promulgated a law that abolishes the death penalty"). A grammar guide not recognizing this conjugation would have advised using a synonym of abolir or using an acceptable form such as ha abolido.
Three other verbs that supposedly aren't conjugated without endings beginning with -i are agredir ("to attack"), balbucir ("to babble"), and blandir ("to brandish").
Additionally, a handful of uncommon verbs are used rarely, if at all, in forms other than the infinitive and past participle; these include aterirse ("to be freezing stiff"), despavorir ("to be terrified"), desolar ("to destroy") and empedernir ("to petrify").
Finally, soler (a verb that has no direct equivalent in English but is roughly translated as "to be usually") is not conjugated in the conditional, future and (according to some authorities) preterite tenses.
3. Verbs such as gustar that frequently are used in the third person followed by the verb's subject and preceded by an object. Gustar is used in sentences such as me gustan las manzanas for "I like apples"; typically the word that is the subject in the English translation becomes the indirect object of the Spanish verb. Other verbs that are usually used in the same way include doler ("to cause pain"), encantar ("to enchant"), faltar ("to be insufficient"), importar ("to matter"), parecer ("to seem"), quedar ("to remain") and sorprender ("to surprise").
These verbs aren't true defective verbs, because they exist in all conjugations, even though they are most common in the third person. The way they are used also doesn't seem to be particularly unusual to native Spanish speakers; they tend initially to be confusing to English speakers learning Spanish because of the way they are translated.