Concepts, with their boundaries intact.

A framework in which the fundamental objects are one-dimensional strings rather than point particles. Its consistency conditions require more than four spacetime dimensions and produce a graviton in the spectrum, which is why it is studied as a candidate theory of quantum gravity.

An eleven-dimensional framework proposed to unify the five consistent ten-dimensional superstring theories, which appear as limits of it. The source map treats the subject as "string/M-theory" throughout rather than separating them.

Spatial dimensions beyond the three we observe, required by the consistency conditions of string and M-theory. They must be hidden from current experiment, either by being very small or by some other mechanism.

The procedure of taking a higher-dimensional theory and rolling up the extra dimensions on a small compact internal manifold, leaving an effective four-dimensional theory. In string theory the internal space is typically a Calabi-Yau manifold.

Compactification with background field strengths (fluxes) threaded through cycles of the internal manifold. The fluxes are quantized, and different choices generate different four-dimensional potentials.

Massless or light scalar fields in the four-dimensional theory that parameterize the size, shape, and coupling of the internal geometry. Left unstabilized they would produce unobserved long-range forces and a time-varying coupling.

The maximally symmetric solution of Einstein gravity with negative cosmological constant. Its geometry has a timelike boundary at spatial infinity.

The principle that a gravitational theory in a region can be equivalently described by a non-gravitational theory on its boundary. The AdS/CFT correspondence is the sharply formulated example, relating gravity in anti-de Sitter space to a conformal field theory on the boundary.

A proposed construction of a de Sitter vacuum in three steps: stabilize complex-structure moduli with fluxes, stabilize the volume with non-perturbative effects into a supersymmetric anti-de Sitter minimum, then uplift that minimum to positive vacuum energy with an anti-D3-brane.

An alternative moduli-stabilization scheme in which the internal volume is stabilized at exponentially large values, nominally suppressing the corrections that could spoil the construction. Some versions uplift using F-terms of blow-up modes rather than anti-branes.

The maximally symmetric solution of Einstein gravity with positive cosmological constant: an exponentially expanding, accelerating spacetime with a cosmological horizon.

The proposed set of effective field theories that look internally consistent as low-energy theories but admit no ultraviolet completion in quantum gravity. The swampland program tries to characterize that set with conjectured criteria.

A constant energy density of the vacuum, denoted Λ, entering Einstein equations as a uniform source. Positive Λ drives accelerated expansion; negative Λ gives anti-de Sitter geometry.

The proposal that scalar potentials in any consistent quantum-gravity effective theory satisfy a gradient bound of the form |∇V| ≥ c·V, with c a positive number of order one in Planck units. Taken at face value it forbids stable de Sitter minima, where the gradient vanishes and V is positive.

A weakened form of the de Sitter conjecture that replaces the strict gradient bound with a disjunction: either the gradient bound holds, or the potential has a sufficiently negative eigenvalue of its Hessian. This permits local maxima while still forbidding stable minima with positive energy.

The empirical study of the universe at large scales, including measurements of the expansion history, the cosmic microwave background, and large-scale structure. In this debate it supplies the dark-energy equation of state and the constraints that any proposal must survive.