calibration curve
A calibration curve is the cheat sheet an instrument uses to turn its raw readings into a real answer. Imagine measuring out cups of coffee of known strength and noting how dark each one looks; later, when a stranger hands you an unknown cup, you compare its darkness against your notes and read off how strong it must be. The chart of "known strength versus how it looked" is the calibration curve.
More precisely, you prepare several standards — solutions whose analyte concentration you already know — measure the instrument's signal for each, and plot signal (on the vertical axis) against concentration (on the horizontal axis). The points usually fall close to a straight line. To find an unknown, you measure its signal and read back across the line to the concentration that would have produced it.
It matters because almost no instrument reports concentration directly; it reports a voltage, an absorbance, a peak area — a signal. The calibration curve is the bridge between that signal and the number you actually want. But the bridge only holds within the range you measured: reading off a concentration far beyond your highest standard is guessing, not measuring.
Five iron standards (0, 2, 4, 6, 8 mg/L) give absorbances of 0.00, 0.20, 0.41, 0.60, 0.81. The points fall on a straight line; an unknown reading of 0.50 corresponds to about 5 mg/L iron.
Known standards define the line; the unknown's signal is read back to its concentration.
A calibration curve is built from external standards measured separately from the sample. When the sample's own background distorts the signal, the standard-addition method — spiking known amounts into the sample itself — is often safer than a plain calibration curve.