An electron occupying a pi-type molecular orbital. In a conjugated planar molecule, the pi subspace is commonly built from p orbitals perpendicular to the molecular plane; its electron population excludes the sigma framework.

A delocalized pi system made from three compatible p orbitals and four electrons. In the simple orbital construction the bonding and nonbonding combinations are occupied and the antibonding combination is empty.

Phase-contrast imaging converts spatial phase changes of light into intensity differences by interference with a reference field. In an off-resonant atomic cloud, the optical phase carries information about column density. Weak probing can support repeated measurements with reduced absorption, although destructiveness still depends on detuning and probe strength.

A phylogenetic tree describing the ancestry of sampled gene copies or a genomic region. It need not match a species tree, because lineage sorting, duplication or introgression can produce a different history.

A representation of the branching history of species lineages. Individual gene trees can differ, so a well-supported tree for one gene is not automatically a complete species history.

A phylogenetic epoch is an interval between successive branching or merging events during which the number of extant ancestral branches is constant. Its expected duration with $k$ branches is denoted $E_k$.

A definition selecting the smallest clade containing specified internal reference taxa: their last common ancestor and every descendant. The specifiers need not be living.

A definition selecting the largest clade containing specified internal reference taxa but excluding specified external reference taxa. With one of each, it includes organisms more closely related to the internal than to the external specifier.

A named organism, species or other specified reference used to anchor a clade definition. Internal specifiers are included and external specifiers excluded.

Photoresist contrast measures how sharply a photoresist changes its developed thickness as the exposure dose changes. For a positive photoresist, let $h(D)$ be the remaining thickness after a specified development process and $h_0$ the initial thickness. A contrast curve plots $h(D)/h_0$ against ${\log }_{10}D$. The measured photoresist contrast is the magnitude of the slope near clearing; it need not describe the entire curve.^[ref]

In an idealized model, the thickness decreases linearly with ${\log }_{10}D$ between a retention exposure dose $D_0$ and a clearing exposure dose $D_c$, where $0<D_0<D_c$. ThenOutside that interval the idealized thickness is clipped to $1$ or $0$. A large $\gamma$ means a smaller exposure dose ratio suffices to change from retention to clearing. This is why photoresist contrast influences the critical modulation for a photoresist; the inverse-logarithm formula assumes this particular contrast-curve model.

Radiant energy per exposed area. For steady intensity $I$ and exposure duration $\tau$, $D=I\tau$; the developed photoresist profile also depends on chemical processing.

A photoresist whose exposed regions become less soluble in developer over the relevant process window, so adequately exposed regions remain after development.

A photoresist whose exposed regions dissolve faster in the developer than unexposed regions.

Selective dissolution of resist after exposure to reveal the written pattern.

Light-dependent movement of chloroplasts within plant cells. Accumulation can increase interception under weak light, whereas avoidance can reduce exposure under strong light.

A model in which a light-sensitive internal phase produces a response when it coincides with the external light interval. Equal total light need not produce equal responses if timing differs.

CONSTANS is an Arabidopsis thaliana transcriptional regulator involved in photoperiodism. Its daily expression pattern and light-dependent protein regulation help activate FLOWERING LOCUS T when internal timing coincides with suitable illumination.

The useful comparison is not merely long days versus short days: changing the phase of light relative to the regulator's availability tests the coincidence mechanism. This explains how a plant can use a daily light pattern as seasonal information.^[ref]