Indicator
An indicator is a substance that changes color depending on the pH of a solution, used to tell whether something is acidic, basic, or neutral.
Definition
An indicator is a special dye, usually a weak acid or weak base itself, that changes color depending on the pH of a solution, letting chemists see acidity or basicity that would otherwise be invisible; litmus shifts from red to blue around pH $7$, phenolphthalein turns from colorless in acid to pink in base, and universal indicator sweeps through a rainbow of colors across the whole scale, with indicators commonly used in titrations to signal the exact moment an acid and base have neutralized each other. Chemically, an indicator is a conjugate acid-base pair, $HIn \rightleftharpoons H^+ + In^-$, whose two forms absorb light differently because protonation changes the extent of conjugation in its chromophore, and its color transition, centered on its own $\text{p}K_{In}$, spans roughly $\text{p}K_{In} \pm 1$ since the eye needs the ratio $[In^-]/[HIn]$ to shift from about $1{:}10$ to $10{:}1$ before it notices a change.
Example
Red cabbage juice is a natural indicator that turns pink in vinegar and green or blue in baking soda water, and adding phenolphthalein to a colorless acid keeps it clear until base added drop by drop reaches the exact point of neutralization, when the whole solution suddenly turns pink; bromothymol blue shifts from yellow below pH $6$ through green to blue above $7.6$, useful for checking aquarium water. Phenolphthalein, with $\text{p}K_{In} \approx 9.4$, changes color between pH $8.2$ and $10.0$, suiting the titration of acetic acid with $NaOH$, whose equivalence point sits near pH $8.7$, while methyl orange, with $\text{p}K_{In} \approx 3.5$, changes far too early for that titration but works well for titrating ammonia with $HCl$.
Key Insight
Tea is its own kind of indicator, since squeezing lemon into it lightens its color because the acid changes the shape of the tea's pigment molecules. An indicator is really just a tiny amount of weak acid added to a solution, so it consumes a negligible amount of titrant, but using too much of it still introduces measurable error, which is why the best indicator is always the least amount you can actually see change color.