Base
In chemistry, a base is a substance that releases hydroxide ions in water or accepts hydrogen ions, feels slippery, turns red litmus blue, and has a pH above 7.
Definition
A base is a substance that produces hydroxide ions ($OH^-$) when dissolved in water or accepts hydrogen ions from an acid, tastes bitter, feels slippery, turns red litmus paper blue, reacts with acids to form water and a salt, and has a pH above $7$; strong bases like sodium hydroxide dissociate completely while weak bases like ammonia react only partly with water. In the Bronsted-Lowry sense a base is any proton acceptor, and in the Lewis sense it is any electron-pair donor, with base strength measured by $K_b$ for $B + H_2O \rightleftharpoons BH^+ + OH^-$; for any conjugate acid-base pair $K_a \times K_b = K_w = 1.0 \times 10^{-14}$ at $25^\circ\text{C}$, so a strong acid leaves behind a negligible conjugate base while a weak acid's conjugate base can be moderately strong, and amines act as bases because nitrogen's lone pair accepts a proton.
Example
Soap, baking soda, and ammonia cleaner are all bases, and antacid tablets are bases that soothe an upset stomach by neutralizing excess acid; sodium hydroxide dissociates as $NaOH \rightarrow Na^+ + OH^-$ and a $0.1\text{ M}$ solution reaches pH $13$, while baking soda solution sits around pH $8.3$, household ammonia near $11$, and bleach around $12.5$. Ammonia has $K_b = 1.8 \times 10^{-5}$, so a $0.10\text{ M}$ solution has $[OH^-] \approx 1.3 \times 10^{-3}\text{ M}$, a pOH of $2.9$, and a pH of $11.1$, and its conjugate acid $NH_4^+$ has $K_a = K_w/K_b = 5.6 \times 10^{-10}$, which is why ammonium salts turn out slightly acidic.
Key Insight
Bases feel slippery because they react with the natural oils on your skin to form a tiny bit of soap, the same saponification reaction that has turned fat and lye into soap for thousands of years, and strong bases like drain cleaner can burn skin just as badly as strong acids, making them among the most dangerous chemicals in an ordinary house. The Lewis definition even makes $BF_3$ an acid and $NH_3$ a base with no proton or hydroxide involved at all, since boron's empty orbital simply accepts nitrogen's lone pair, and most of coordination chemistry and catalysis is acid-base chemistry in exactly this broader sense.