Correlation
Correlation measures how closely two assets move together, from −1 (opposite) through 0 (unrelated) to +1 (in step).
The usual measure compares two coins' daily returns over a period. Most large cryptocurrencies are strongly correlated with Bitcoin, which means holding several of them diversifies less than it looks, especially in a sell-off, when correlations tend to rise.
Correlation changes over time and says nothing about cause. See the crypto correlation matrix for how closely the largest coins have moved together over 30 days, 90 days, a year and all time.