Acid
An acid is a substance that releases hydrogen ions in water, tastes sour, turns blue litmus red, and has a pH below 7; examples include vinegar and lemon juice.
Definition
An acid is a substance that releases hydrogen ions ($H^+$) when dissolved in water, tastes sour, turns blue litmus paper red, reacts with many metals to release hydrogen gas, reacts with bases to form water and a salt, and has a pH below $7$; strong acids like hydrochloric acid ionize completely in water while weak acids like acetic acid ionize only partly. In the broader Bronsted-Lowry definition an acid is any proton donor, and in the even broader Lewis definition it is any electron-pair acceptor, and acid strength is quantified by the acid dissociation constant $K_a$ for $HA \rightleftharpoons H^+ + A^-$, or by $\text{p}K_a = -\log K_a$, with a lower $\text{p}K_a$ meaning a stronger acid, depending on the polarity and strength of the $H$-$A$ bond and the stability of the conjugate base $A^-$.
Example
Lemon juice, vinegar, and the citric acid in soda are acids, and your stomach makes hydrochloric acid to digest food; hydrochloric acid ionizes as $HCl \rightarrow H^+ + Cl^-$, vinegar is about $5\%$ acetic acid ($CH_3COOH$) with a pH near $2.5$, and stomach acid runs from pH $1.5$ to $3.5$. Acetic acid has $K_a = 1.8 \times 10^{-5}$, so a $0.10\text{ M}$ solution has $[H^+] \approx 1.3 \times 10^{-3}\text{ M}$ and a pH near $2.9$, noticeably milder than $0.10\text{ M}$ $HCl$, which is fully ionized and has a pH of $1.0$, and sulfuric acid is diprotic, donating two protons in sequence with very different $K_a$ values for each.
Key Insight
Never taste-test a chemical to find out whether it is an acid, since strong acids like battery acid cause serious burns even though many safe, weak acids do taste sour. Strong and concentrated are different ideas entirely: a strong acid ionizes completely regardless of how much is dissolved, so dilute hydrochloric acid is still a strong acid and concentrated vinegar is still a weak one, and even water itself acts as a Bronsted-Lowry acid when it donates a proton to ammonia to form $NH_4^+$ and $OH^-$, which shows acidity is a relationship between a donor and an acceptor rather than a fixed label on a molecule.