# Concepts
URL: /docs/concepts

The vocabulary Tachyon uses — collections, documents, fields, segments, and more.



## Collection [#collection]

A named group of documents sharing one schema — roughly equivalent to a table
or an index in other systems. See [Collections](/docs/collections).

## Document [#document]

A single searchable record: a flat JSON object. Every document has an `id`,
implicit and never declared as a field. Field names starting with `_` are
reserved. See [Documents](/docs/documents).

## Field [#field]

A named, typed value on a document, declared on the collection's schema —
`text`, `keyword`, `int`, `float`, `bool`, or `date`. A field is only
filterable, sortable, or facetable if the schema says so explicitly; text is
always searched.

## Inverted index [#inverted-index]

The data structure that maps tokens to the documents containing them, built
as documents are added — what makes full-text search fast, because a query
looks up a short list of matching documents instead of scanning every one.
See [Inverted Index](/docs/concepts/inverted-index) for how Tachyon's is
laid out on disk.

## Token [#token]

The unit the tokenizer produces from text: roughly, a run of alphanumeric
characters, lowercased and Unicode-normalized. Tachyon does not stem words or
strip stop words — `"running"` and `"run"` are different tokens. See
[Tokenization](/docs/concepts/tokenization) for the exact rules.

## Ranking [#ranking]

Search results are scored with BM25, a standard lexical relevance function
that rewards term frequency in a document while discounting terms that are
common across the whole collection. See [Relevance & BM25](/docs/relevance-bm25).

## Block-max WAND [#block-max-wand]

The query execution strategy that skips postings blocks provably incapable
of ranking in the top results, without changing which documents end up
there. See [Block-max WAND](/docs/concepts/block-max-wand).

## Filter [#filter]

A boolean expression over fields declared `filter: true` (or `facet`/`sort`)
that narrows a search to matching documents without affecting ranking. See
[Filtering](/docs/filtering).

## Facet [#facet]

A count of how many matching documents hold each value of a field declared
`facet: true` — the data behind a "Brand (12) / Category (8)" style sidebar.
See [Faceting](/docs/faceting).

## Write-ahead log [#write-ahead-log]

The append-only, checksummed log every write hits before it's acknowledged,
replayed on startup to recover anything not yet flushed to a segment. See
[Write-Ahead Log](/docs/concepts/write-ahead-log).

## Segment [#segment]

An immutable, memory-mapped chunk of a collection's index on disk. New
documents accumulate in memory (a *memtable*) and are periodically flushed
into a new segment; segments are later folded together by a background
merge. See [Segment Merging](/docs/concepts/segment-merging) and
[Persistence](/docs/persistence).