What is an index alias in Elasticsearch?

An alias is a secondary name that points to one or more indices, allowing you to decouple your application from the physical index name. For read/write separation, you can have a read alias pointing to multiple indices and a write alias pointing only to the current active index. The most powerful use case is zero-downtime reindexing: your application always reads/writes via the alias, so when you need to reindex, you atomically swap the alias from old to new with a single POST /_aliases call (no application code changes). Aliases can also include a filter query to create a virtual "view" of a subset of documents.