Metric (度量) $d: X^2 \to \mathbb{R}_{\ge 0}$ on a topological space $(X, \mathcal{T})$ is a non-negative bivariate function that satisfies:
Psudo-metric is a mapping satisfying all the requirements of a metric except non-degeneracy. For example, for the space $C(\mathbb{R})$ of continuous real functions, $\rho_n(f, g) = \sup_{x \in [-n,n]} |f(x) - g(x)|$, $n \in \mathbb{N}$, are psudo-metrics. But they can be transformed into a metric, e.g. $\sigma(f, g) = \sum_{n=1}^{\infty} 2^{-n} \min \{1,\rho_n(f, g) \}$.
Metric space (度量空间) $(X, d)$ is a set $X$ with a metric $d: X \times X \to \mathbb{R}_{\ge 0}$. Metric specifies the distance among elements within a space. Subspace $(A, d)$ of a metric space $(X, d)$, where $A \subset X$, is also a metric space. Product space $(X \times Y, d_x \times d_y)$ of two metric spaces $(X, d_x)$ and $(Y, d_y)$, where $d_x \times d_y ((x_1, y_1), (x_2, y_2)) = d_x(x_1, x_2) + d_y(y_1, y_2)$, is also a metric space.
Distance between a point and a set in a metric space $(X, d)$ is the mapping $d: X \times \mathcal{P}(X) \to \mathbb{R}{\ge 0}$ such that $d(x, A) = \inf{y \in A} d(x,y)$. Diameter of a set in a metric space $(X, d)$ is the mapping $\mathrm{diam}: \mathcal{P}(X) \to \mathbb{R}_{\ge 0}$ such that $\mathrm{diam}(A) = \sup_{x,y\in A} d(x,y)$.
Bounded space is a metric space whose diameter is finite. Totally-bounded space is a metric space that can be represented as a finite union of arbitrarily bounded subspaces: $\forall \varepsilon > 0$, $\exists \{x_i\}_{i=1}^{n} \subset X$: $X \subset \cup_{i=1}^n B_\varepsilon(X_i)$.
Lp metric is the metric induced from an Lp norm.
Euclidean metric...
Isometry (等距同构) or congruent transformation is a distance-preserving bijective map between two metric spaces: given metric spaces $(X, d_x)$ and $(Y, d_y)$, bijection $f: X \to Y$ is an isometry iff $d_x(x_1, x_2) = d_y(f(x_1), f(x_2)), \forall x_1, x_2 \in X$. For example, bending a plane is an isometry. Two metric spaces are isometric if and only if there exists an isometry between them.
Convergent sequence is a sequence $\{x_n\}_{n \in \mathbb{N}}$ in a metric space $(X, d)$ such that $\exists x \in X$, $\forall \varepsilon > 0$, $\exists N \in \mathbb{N}:$ $\forall n > N$, $d(x_n, x) < \varepsilon$; and we say the sequence converges to $x$. A sequence in a set may converge in one metric but not in another.
Uniform convergence.
Cauchy sequence is a sequence $\{a_i\}_{i=1}^N$ in a metric space that satisfies: $\forall \varepsilon > 0$, $\exists N \in \mathbb{N}:$, $\forall m, n > N$, $d(a_m, a_n) < \varepsilon$.
Complete space is a metric space where every Cauchy sequence converges.
Theorem: (equivalence of metric and topological definitions of complete space).
Baire's Theorem.
Contraction mapping theorem.
Completion.
Equivalent sequence.
Theorem: (Completion of metric space).
Open ball $B_r(x)$ of radius $r$ centered at $x$ in a metric space $(X, d)$ is the set of points with distance to $x$ less than $r$: $B_r(x) = \{y \in X \mid d(y, x) < r\}$, $r > 0$. Closed ball $B_r\left[x\right]$ is the set of points with distance to $x$ no greater than $r$: $B_r\left[x\right] = \{y \in X \mid d(y, x) \le r\}$, $r \ge 0$. Sphere $S_r\left[x\right]$ is the set of points with distance to $x$ equal $r$: $S_r\left[x\right] = \{y \in X \mid d(y, x) = r\}$, $r \ge 0$.
Topology generated by a metric or metric topology $\mathcal{T}_d$ is the topology $\mathcal{T(B)}$ generated by the class $\mathcal{B}$ of open balls in a metric space $(X, d)$: $\mathcal{T}_d = \mathcal{T(B)}$, $\mathcal{B} = \{B_r(x) \mid x \in X, r > 0\}$. Euclidean topology, usual topology, or ordinary topology on $\mathbb{R}^n$ is the topology generated by the Euclidean metric in $\mathbb{R}^n$.
Sequentially compact.
Boltzano-Weierstras property
Covering, sub-covering, open covering, compact.
Covering number $N(S,s)$ of a metric space $S$ with disks at radius $s$ is the minimal number of such disks needed to cover the space. This number is finite if the metric space is compact.
Compactness Theorem: equivalence of sequentially compact, compact, Boltzano-Weierstras property, H-compact
Theorem: continuous mapping from a compact space induces another compact space.
Theorem: product space of compact spaces is compact.
Boltzano-Weierstras theorem
Theorem: real continuous mapping from a compact space has maximum and minimum.
Pointwise compace, equi-continuous.
Theorem: (Arzela-Ascoli)
In case of function spaces, compact spaces typically are similar to finite-dimensional space.
Theorem: (equivalence of metric and topological definitions of continuous mapping)
continuity, uniform continuity.