adj. I. Of or pertaining to the world.

1. Eccl. a. Of members of the clergy: Living ‘in the world’ and not in monastic seclusion, as distinguished from ‘regular’ and ‘religious’. secular canon: see canon n.2 secular abbot: a person not a monk, who had the title and part of the revenues, but not the functions of an abbot.
In early use frequently placed after the n., as canon secular, priest secular.
b. Of or pertaining to secular clergy.

2. a. Belonging to the world and its affairs as distinguished from the church and religion; civil, lay, temporal. Chiefly used as a negative term, with the meaning non-ecclesiastical, non-religious, or non-sacred.

†b. transf. Of or belonging to the ‘common’ or ‘unlearned’ people. Obs.

c. Of literature, history, art (esp. music), hence of writers or artists: Not concerned with or devoted to the service of religion; not sacred; profane. Also of buildings, etc., Not dedicated to religious uses.

d. Of education, instruction; Relating to non-religious subjects. (In modern use often implying the exclusion of religious teaching from education, or from the education provided at the public expense.) Of a school: That gives secular education.

3. a. Of or belonging to the present or visible world as distinguished from the eternal or spiritual world; temporal, worldly. Also secular-minded adj.

b. Caring for the present world only; unspiritual. rare.

¶4. Used for: Pertaining to or accepting the doctrine of secularism; secularistic.
secular societies: the designation given to associations formed in various English towns from 1852 onwards to promote the spread of secularist opinions.

II. Of or belonging to an age or long period.

5. Occurring or celebrated once in an age, century, or very long period. secular games, plays, shows [L. ludi sæculares]: in ancient Rome, games continuing three days and three nights celebrated once in an ‘age’ or period of 120 years. secular poem [L. carmen sæculare], a hymn composed to be sung at the secular games.

6. Living or lasting for an age or ages. Now chiefly with reminiscence of the scientific sense 7. Also (of trees, etc., after F. séculaire), centuries old.

7. In scientific use, of processes of change: Having a period of enormous length; continuing through long ages. a. Astr. Chiefly of changes in the orbits or the periods of revolution of the planets, as in secular acceleration, equation, inequality, variation. The terms secular acceleration, secular variation were formerly also used (with reference to the sense ‘century’ of L. sæculum) for the amount of change per 100 years; similarly †secular precession (see quot. 1812). secular equation is also used more widely to designate any equation of the form |aij-bijk| = 0 (i,j = 1,2, . . ., n), in which the left-hand side is a determinant and which arises in quantum mechanics.
1801 Monthly Rev. XXXV. 537 M. De La Place+found the secular equation of the moon to be due to the action of the sun on the moon. 1812 Woodhouse Astron. ix. 63 The secular precession, that is, the accumulated precessions of 100 years. 1812–16 Playfair Nat. Phil. (1819) II. 275 In the orbit of Mars, the eccentricity is diminishing. The secular variation of the greatest equation of the centre is—37". 1834 M. Somerville Connex. Phys. Sci. iii. (1849) 16 Secular inequalities. 1862 Cayley Math. Papers (1890) III. 522 On the Secular Acceleration of the Moon's Mean Motion. 1937 E. C. Kemble Fund. Princ. Quantum Mech. x. 361 Its components must yield a nontrivial (i.e., nonvanishing) solution of the set of g equations Rn(Amn - admn)xn = 0.+ Such a solution exists only if the determinant of the coefficients vanishes, i.e., if a is a root of the so-called ‘secular’ equation det (A - aI) = +0. 1974 Gill & Willis Pericyclic Reactions i. 21 To obtain the wave functions corresponding to these energies it is necessary to solve the secular equations using the appropriate values of E.

b. Geol., Physical Geogr., Meteorol., etc.
1833 Lyell Princ. Geol. Gloss., Secular Refrigeration, the periodical cooling and consolidation of the globe, from a supposed original state of fluidity from heat. 1856 Kane Arct. Expl. I. xxiii. 308 A secular elevation of the coastline. 1861 Tyndall Fragm. Sci. xiii. (1871) 399 The earth's magnetic constituents are gradually changing their distribution. This change is very slow; it is technically called the secular change. 1867 H. Macmillan Bible Teach. xvi. (1870) 320 Those grand secular tides which have punctually recurred every ten thousand years. 1872 I True Vine v. 176 The earth has its secular seasons as well as its annual. 1880 Haughton Phys. Geog. ii. 53 The contraction of the globe due to secular cooling. 1887 Abercromby Weather 312 Annual and Secular Variations.


so... secular equilibrium is that established over a loooong time period, as opposed to transient equilibrium.