A defective script is a writing system that does not represent all the phonemic distinctions of a language. It is different from an irregular script, which can distinguish all the phonemes of the language even if in practice it does not always do so.
For example, Italian has seven vowels, but the Italian alphabet has only five vowel letters to represent them; in general, the differences between /e, ɛ/ and /o, ɔ/ are simply ignored, though when stress marks are used they may distinguish them. Among the consonants, both /s/ and /z/ are written ⟨s⟩, and both /ts/ and /dz/ are written ⟨z⟩, though not many words are distinguished by the latter. Stress and hiatus are not reliably distinguished.
Such imperfections are nothing new. The Greek alphabet was defective during its early history. Classical Greek had distinctive vowel length: five short vowels, /i e a o u/, and seven long vowels, /iː eː ɛː aː ɔː oː uː/. When the Phoenician alphabet was adapted to Greek, the names of five letters were pronounced with initial vowels by the Greeks and used acrophonically to represent vowels. These were alpha, e (later called e psilon), iota, o (later called o micron, and u (later called u psilon): five letters for twelve vowel sounds. Later the dropped from the Eastern Greek dialects, and the letter heta (now pronounced eta) became available; it was used for /ɛː/. About the same time the Greeks created an additional letter, omega, probably by writing omicron with an underline, that was used for /ɔː/. Digraphs ei and ou were devised for /eː/ and /oː/. Thus Greek entered its classical era with seven letters and two digraphs for twelve vowel sounds. Long /iː aː uː/ were never distinguished from short /i a u/, even though the distinction was meaningful. Although the Greek alphabet was a good match to the consonants of the language, it was defective when it came to some vowels.
A famously defective script is the Arabic one. The modern script does not normally write short vowels, but for the first few centuries of the Islamic era, long vowels were not written and many consonant letters were ambiguous as well. The Arabic script derives from the Aramaic, and not only did the Aramaic language have fewer phonemes than Arabic, but several originally distinct Aramaic letters had conflated (become indistinguishable in shape), so that in the early Arabic writings 18 letters had to do duty for 28 consonant phonemes — and in the middle of words, only 15 were distinct. For example, medial ⟨ﯩ⟩ represented /b, t, θ, n, j/, and ⟨ﺡ⟩ represented /ɡ, ħ, x/. A system of diacritic marks, or pointing, was later developed to resolve the ambiguities, and over the centuries became nearly universal. However, even today unpointed texts of a style called mašq are found, where these consonants are not distinguished.
Without short vowels or geminate consonants being written, modern Arabic نظر nẓr could represent /naðˤara/ 'he saw', /naðˤːara/ 'he compared', /nuðˤira/ 'he was seen', /nuðˤːira/ 'he was compared', /naðˤar/ 'a glance', or /niðˤr/ 'similar'. However, in practice there is little ambiguity, as the vowels are more easily predictable in Arabic than they are in a language like English. Moreover, the defective nature of the script has its benefits: the stable shape of the root words, despite grammatical inflection, results in quicker word recognition and therefore faster reading speeds; and the lack of short vowels, the sounds which vary the most between Arabic dialects, makes texts more widely accessible to a diverse audience.
However, in mašq and those styles of kufic writing which lack consonant pointing, the ambiguities are more serious, for here different roots are written the same. ﯨطر could represent the root nẓr 'see' as above, but also nṭr 'protect', bṭr 'pride', bẓr 'clitoris' or 'with flint', as well as several inflections and derivations of each of these root words.
When a defective script is written with diacritics or other conventions to indicate all phonemic distinctions, the result is called plene writing.
Defectiveness is a cline: The Semitic abjads do not indicate (all) vowels, but there are also alphabets which mark vowels but not tone. Even if English orthography were regularized, the English alphabet would still be incapable of unambiguously conveying intonation, though since this is not expected of scripts, it is not normally counted as defectiveness.