Evolution of Latin /f/ in Spanish

The historical change by which Latin word-initial /f/ was generally aspirated and later lost in Spanish, though it survived in certain phonological environments and words.

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Latin /f/: The voiceless labiodental fricative /f/ in Latin, including its use at the beginnings of words. Spanish inherited this sound and changed it most conspicuously at word beginnings.
Hacer: The Spanish verb meaning “to do” or “to make,” descended from Latin facere. Its initial h reflects the loss of Latin f in a common inherited word.
Vulgar Latin: The diverse spoken forms of Latin from which the Romance languages developed. The Spanish change emerged in the spoken Latin varieties ancestral to Spanish.
Latin /h/: The breathy consonant represented by h in some Latin words, distinct from Latin /f/. The Spanish letter h often reflects lost /f/, not a direct continuation of Latin /h/.
Phonological lenition: A sound change in which a consonant becomes less constricted or otherwise weaker. Aspiration and eventual deletion are stages of weakening in many accounts of the change.
Hijo: The Spanish noun meaning “son,” descended from Latin filius. Its silent initial h represents the familiar f-to-h-to-zero outcome.
Spanish language: A Romance language that developed from spoken Latin in the Iberian Peninsula. This sound change is one strand in Spanish’s historical development from Latin.
Latin /b/ and /v/: Latin consonants whose later Romance outcomes differed by position and regional development. Their histories show why Spanish retention or loss cannot be inferred from spelling alone.
Glottal fricative: A fricative articulated at the glottis, commonly represented as /h/. The intermediate aspiration is often reconstructed as a glottal fricative.
Humo: The Spanish noun meaning “smoke,” descended from Latin fumus. It illustrates loss of initial Latin f in a word with a following back vowel.