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Stock Market Terms Glossary: From P/E to Circuit Breakers

From basic valuation ratios to the mechanisms that pause trading during a crash, here are the stock market terms worth actually understanding.

P/E Ratio (Price-to-Earnings)

A stock's price divided by its earnings per share. A lower P/E is often read as cheaper relative to profits, but the 'right' level varies enormously by industry and growth rate, so it only means something when compared against peers or the company's own history.

Short Selling

Borrowing shares to sell immediately, hoping to buy them back later at a lower price and pocket the difference. Profits are capped at 100% (if the stock goes to zero), but losses are theoretically unlimited since a stock can keep rising β€” and a rush of shorts buying back at once can trigger a short squeeze.

Dead Cat Bounce

A brief, sharp price recovery in the middle of a sustained downtrend. It is easy to mistake for a reversal, but the decline typically resumes once the bounce runs out of buyers.

Circuit Breaker

A market-wide trading halt triggered when a major index falls a set percentage in a single session. In the US, S&P 500 declines of 7%, 13%, and 20% each trigger a tiered response, from a 15-minute pause to closing the market for the day.

Limit Up / Limit Down (LULD)

A safeguard for individual stocks (as opposed to the whole market) that pauses trading in a single name when its price moves too far, too fast within a short window β€” distinct from a market-wide circuit breaker.

Market Capitalization

Share price multiplied by total shares outstanding. It's the standard way stocks get grouped into large-cap, mid-cap, and small-cap categories.

Why exchanges pause trading at all

Circuit breakers and LULD pauses exist to give investors a moment to absorb new information instead of reacting purely to momentum and panic. The US market-wide circuit breaker system traces back to reforms after the 1987 "Black Monday" crash, when the absence of any brake contributed to a self-reinforcing selloff.

A P/E ratio only means something in context

A P/E number in isolation tells you almost nothing β€” it needs to be compared against the industry average or the company's own historical range. A high P/E can mean a stock is overpriced, or it can simply reflect strong growth expectations; a cyclical company's P/E can look artificially high or low right around an earnings swing.

Frequently Asked Questions

Is short selling illegal?

No β€” it is a legal, regulated strategy in most major markets. It is heavily monitored, though, and regulators sometimes impose temporary restrictions on short selling during periods of extreme volatility.

What exactly triggers a US market-wide circuit breaker?

A 7% intraday drop in the S&P 500 triggers a 15-minute market-wide pause, a 13% drop triggers another 15-minute pause, and a 20% drop closes the market for the remainder of the trading day.