Expanding the relativistic kinetic energy $\sqrt{m^2{c}^4+c^2{p}^2}-m{c}^2$ for $|p|\ll mc$ gives $p^2/(2m)-p^4/(8m^3{c}^2)+\cdots$. The first correction is negative and contributes to atomic fine structure. Its perturbative use requires small momentum compared with mc; the truncated negative quartic term is not a globally stable exact Hamiltonian.

The spin-one-half low-energy expansion of the Dirac equation includes this local correction in a static potential energy V. For $V=-Z{e}^2/(4\pi {ϵ}_{0}r)$, ${\nabla }^{2}V=Z{e}^2{\delta }^{(3)}(\mathbf{r})/{ϵ}_0$, so the correction is positive and probes the wavefunction at the origin. In point-Coulomb hydrogen atom states it contributes only to S waves at leading nonrelativistic order. It is part of fine structure, distinct from the radiative Lamb shift. ^[ref]

For a point, fixed proton, the sum of the relativistic kinetic-energy correction, spin-orbit interaction including the Thomas precession factor, and Darwin term has the displayed leading correction. It depends on n and total angular momentum j, but not separately on l. Thus $2S_{1/2}$ and $2P_{1/2}$ remain degenerate while $2P_{3/2}$ lies above them. Radiative corrections, recoil, nuclear size and hyperfine structure require additional terms. The fine-structure constant is ${\alpha }_{\mathrm{fs}}=e^2/(4\pi {ϵ}_0\hslash c)$.

A fin ray is a supporting element within a fin. Its structure differs among lineages; bony rays and cartilaginous or other supporting materials should not be assumed identical. Flexible arrangement of rays permits control of fin shape during movement.

A member of a bilateral appendage pair. Pectoral and pelvic fins have distinct positions and need not have originated simultaneously.

An unpaired fin on the dorsal or ventral midline, including dorsal, anal and Caudal appendages.

A region with a consistent orientation of spontaneous electric polarization. Antiparallel domains can give opposite signs of piezoresponse force microscopy response.

Application of an electric field to favour ferroelectric domains whose polarization aligns with the field, leaving a net remanent polarization. In a ceramic the crystal grains remain differently oriented; domain switching does not align their crystallographic axes.

Microbial digestion in an enlarged compartment before the principal acid and enzymatic digestive regions. Independent specializations occur in ruminants, colobine monkeys and macropods.

Microbial digestion in enlarged caecal or colonic regions after the principal small-intestinal digestive region. Intake, particle sorting and retention affect its consequences for nutrition.

A very short exon. Specialized alignment and junction validation can be needed to detect its inclusion; the exact length cutoff should be stated for a given analysis.

A proposed origin of a bisexual reproductive structure through regional modification of a formerly male structure. In one developmental scenario, restriction of B-class activity permits female identity in part of a C-active reproductive axis. Accessibility in a model system does not establish the historical precursor.

A specific historical hypothesis in which ectopic ovules arose on a predominantly male precursor during flower evolution. It combines gene-history and morphological arguments but remains distinct from a demonstrated ancestral developmental sequence.^[ref]

The classical avian hypothesis that anatomical digit identities I, II and III are expressed at positional condensations II, III and IV. It distinguishes position from developmental identity; evidence for a limited anterior shift does not require invariant identities for every posterior anatomical digit.^[ref]

Evolutionary change in the amount or level of a developmental process, structure or expression pattern. A quantitative difference still requires a causal molecular and developmental explanation.