Terminology

Term	Meaning
Symbol / root / underlying	The unique identifier of a stock, index, or option underlying (e.g. AAPL, SPX, SPXW). The three words are interchangeable.
Contract	One option, identified by symbol + expiration + strike + right. See Symbology.
Right	The option side: C (call) or P (put).
Expiration	The contract's expiration date, written YYYYMMDD.
Strike	The contract's strike price, in dollars on every SDK surface.
NBBO	National Best Bid and Offer — the consolidated best quote across exchanges.
OPRA	Options Price Reporting Authority — the consolidated feed for US option trades and quotes.
SIP	Securities Information Processor — the consolidated feeds for US equities (CTA and UTP).
EOD	End-of-day report: one row per trading day with OHLC, volume, count, and the closing quote.
Snapshot	The latest value (quote, trade, OHLC, …) for the current session, served in real time.
Tick	One typed row of market data — a trade print, a quote update, a bar.
Tier	Your ThetaData subscription level per asset class: Free, Value, Standard, or Pro. See Subscriptions.
FPSS	ThetaData's streaming service — the upstream source behind Streaming.
MDDS	ThetaData's market-data delivery service — the upstream source behind historical requests.