In an alphabetic writing system, a silent letter is a letter that, in a particular word, does not correspond to any sound in the word's pronunciation. In linguistics, a silent letter is often symbolised with a null sign U+2205 ∅ EMPTY SET, which resembles the Scandinavian letter Ø. A null or zero is an unpronounced or unwritten segment.

One of the noted difficulties of English spelling is a high number of silent letters. Edward Carney distinguishes different kinds of "silent" letters, which present differing degrees of difficulty to readers.

The distinction between "endocentric" digraphs and empty letters is somewhat arbitrary. For example, in such words as little and bottle, one might view ⟨le⟩ as an "endocentric" digraph for /əl/, or view ⟨e⟩ as an empty letter; similarly, with ⟨bu⟩ or ⟨u⟩ in buy and build.

Not all silent letters are completely redundant:

Silent letters arise in several ways:

Since accent and pronunciation differ, letters may be silent for some speakers, but not others. In non-rhotic accents, ⟨r⟩ is silent in such words as hard, feathered; in h-dropping accents, ⟨h⟩ is silent. A speaker may or may not pronounce ⟨t⟩ in often, the first ⟨c⟩ in Antarctic, ⟨d⟩ in sandwich, etc.

In the US, the h in herb is silent (an herb), but in the UK, it is pronounced (a herb). The same is true for the l in solder.

In parts of the UK, the a in dictionary and secretary is silent, but in the US, it is pronounced.