# Glossary Short definitions of the terms used throughout this guide. ## Data grid A scrollable, sortable table of rows and columns rendered in a user interface. Perspective's data grid, the [Datagrid plugin](https://www.npmjs.com/package/@perspective-dev/viewer-datagrid), is _virtualized_: it renders only the visible cells, so its cost is independent of the number of rows. ## Pivot table A table which groups rows by one or more columns (row pivots), optionally splits them across the distinct values of other columns (column pivots), and shows an aggregate in each cell. In Perspective, row pivots are `group_by` and column pivots are `split_by`. See [Grouping and Pivots](./explanation/view/config/grouping_and_pivots.md). ## Streaming pivot table A pivot table whose groups and aggregates are updated incrementally as rows are inserted, updated or removed, rather than recomputed. See [Streaming pivot tables](./use_cases/streaming_pivot_table.md). ## `Table` Perspective's columnar, typed data store. A [`Table`](./explanation/table.md) is created from a schema or a dataset (Apache Arrow, CSV, JSON, or a DataFrame in Python), and modified with `update()`, `remove()`, `clear()` and `replace()`. ## `View` A continuous query over a `Table`: a combination of `group_by`, `split_by`, `columns`, `aggregates`, `filter`, `sort` and `expressions`. A [`View`](./explanation/view.md) stays current as its `Table` changes and notifies `on_update` subscribers. ## `group_by` The columns whose distinct values become the rows of a pivot. Multiple levels form an expandable tree with subtotals. ## `split_by` The columns whose distinct values become the column headers of a pivot. ## Aggregate The function which reduces a group's values to one cell — `sum`, `avg`, `count`, `distinct count`, `median`, `weighted mean`, `first`, `last` and others. Chosen per column. ## Expression column A computed column defined in Perspective's [expression language](./explanation/view/config/expressions.md), based on ExprTK. Expressions are evaluated column-wise inside the engine and can be grouped, filtered, sorted and aggregated like stored columns. ## Index A `Table` option naming a primary key column. Updates to an indexed table replace the row with the matching key (and may be partial); without an index, updates append. See [`index` and `limit`](./explanation/table/options.md). ## Limit A `Table` option which keeps only the most recent _n_ rows, for rolling windows over unbounded streams. ## `page_to_disk` A `Table` option which backs the table's columns with on-disk storage — the Origin Private File System in the browser, memory-mapped files natively — so a `Table` can exceed the engine's memory. See [`page_to_disk`](./explanation/table/options.md#page_to_disk). ## `` The Web Component (Custom Element) providing Perspective's user interface: configuration panel, plugins, themes, multi-panel layout and save/restore. ## Plugin A visualization hosted by `` — the Datagrid, or one of the WebGL charts (bar, line, area, scatter, heatmap, treemap, sunburst, candlestick, OHLC, maps). ## Client, Server A `Server` owns tables and executes queries. A `Client` is a handle to a `Server`, whether that server is in the same process, in a Web Worker, or across a WebSocket. The API is the same in every case. ## Client-only mode The engine runs in the browser as WebAssembly in a Web Worker; no server is involved. See [Client-only](./explanation/architecture/client_only.md). ## Client/server replicated mode A server owns the authoritative `Table`; each browser keeps a synchronized copy and queries it locally. See [Client/Server replicated](./explanation/architecture/client_server.md). ## Server-only mode Queries run on the server and the browser receives only the rows it is displaying. See [Server only](./explanation/architecture/server_only.md). ## Virtual server An implementation of Perspective's protocol over an external query engine, such as DuckDB, ClickHouse, PostgreSQL or Polars, which translates view configurations into that engine's native queries. See [Virtual Servers](./explanation/virtual_servers.md). ## Virtual scrolling Rendering only the rows and columns currently in the viewport, and fetching more as the user scrolls. ## Apache Arrow A language-independent columnar memory format. It is Perspective's preferred interchange format: it loads without parsing and preserves types. ## WebAssembly A portable binary instruction format which runs at near-native speed in browsers. Perspective's C++ query engine and Rust UI are both compiled to WebAssembly. ## Memory64 A WebAssembly extension for 64-bit memory addressing. Perspective ships an optional Memory64 engine build which raises the in-browser heap limit from 4GB to 16GB.