A number estimating playing strength from results against other rated players. Both sites use Glicko-2-style systems; a win against a stronger player gains more than a win against a weaker one.
Rating pool
The group of players a rating is measured against. Each site and each time control is its own pool: Lichess blitz, chess.com rapid, and so on. Numbers from different pools aren't comparable, which is why the ladder never mixes them.
Provisional rating
A rating the site flags as uncertain, after few games or a long break. Lichess marks it with a question mark; chess.com shows it after few games or with high rating deviation. It can swing by a lot from one game to the next.
Rating deviation (RD)
How uncertain the site is about a rating. It shrinks as you play and grows when you stop.
Bullet
Games under 3 minutes per player.
Blitz
3 to 8 minutes per player on Lichess (clock time plus 40 times the increment), 3 to 9 on chess.com. The two sites don't cut at exactly the same point.
Rapid
Around 8 to 25 minutes per player on Lichess, 10 minutes or more on chess.com.
Classical
Lichess only: 25 minutes or more per player. chess.com counts long games as rapid.
Daily or correspondence
Days per move. chess.com calls it daily, Lichess correspondence. Shown as one row.
Change
Rating now minus the last rating before the period.
Streak
Days in a row with at least one rated game on either site.
Verified
The player proved the account with a code in their profile.
Benchmark
How players who started at a similar rating changed over 30 days, from every account I track. Shown once 20 players share a 200-point band.
Season
A stretch of time the coach names and ends; its standings are archived.