In linguistics, a filler, filled pause, hesitation marker or planner is a sound or word that participants in a conversation use to signal that they are pausing to think but are not finished speaking. (These are not to be confused with placeholder names, such as thingamajig, whatchamacallit, whosawhatsa and whats'isface, which refer to objects or people whose names are temporarily forgotten, irrelevant, or unknown.) Fillers fall into the category of formulaic language, and different languages have different characteristic filler sounds. The term filler also has a separate use in the syntactic description of wh-movement constructions.
Every conversation involves turn-taking, which means that whenever someone wants to speak and hears a pause, they do so. Pauses are commonly used to indicate that someone's turn has ended, which can create confusion when someone has not finished a thought but has paused to form a thought; in order to prevent this confusion, they will use a filler word such as um, er, or uh. The use of a filler word indicates that the other person should continue listening instead of speaking.
Filler words generally contain little to no lexical content, but instead provide clues to the listener about how they should interpret what the speaker has said. The actual words that people use may change (such as the increasing use of like), but the meaning and reason why people use them does not change.
In American English, the most common filler sounds are ah or uh /ʌ/ and um /ʌm/ (er /ɜː/ and erm /ɜːm/ in British English). Among younger speakers, the fillers "like", "you know", "I mean", "okay", "so", "actually", "basically", and "right?" are among the more prevalent. Christopher Hitchens described the use of the word "like" as a discourse marker or vocalized pause as a particularly prominent example of the "Californianization of American youth-speak", and its further recent spread throughout other English dialects via the mass-media.
The linguistic term "filler" has another, unrelated use in syntactic terminology. It refers to the pre-posed element that fills in the "gap" in a wh-movement construction. Wh-movement is said to create a long-distance or unbounded "filler-gap dependency". In the following example, there is an object gap associated with the transitive verb saw, and the filler is the wh-phrase how many angels: "I don't care [how many angels] she told you she saw."