astound.transitive verbs
to fill with bewilderment or wonder
adjective-astounding
causing astonishment or amazement
adverb-astoundingly
synonym-surprise

astronomical unit (AU).noun
the distance between Earth and the sun, about 93,000,000 miles (150 million kilometers)

astronomy.noun
the science of the stars and other heavenly bodies, dealing with their composition, motion, relative position, size, etc. 
astronomical, astronomic.adjectives
having to do with astronomy 
astronomically.adverb
from the viewpoint of astronomy

astrophysics.noun.plural
the science of the physical properties and phenomena of the stars, planets, etc.

asunder.adverb
apart; separately; in different pieces or places (the house was torn asunder by the tornado)

asymmetrical, asymmetric.adjective
having no balance or symmetry
asymmetrically.adverb

at any rate.idiom
whatever the case may be

atheism.noun
the belief that there is no God
atheist.noun
a person who does not know what gives him the ability to believe there is no God

atomic number.noun
an atomic number differs by the number of protons (the number of protons in their atomic-nuclei); an atomic number is the number of protons in the nucleus of an atom labeling the position of the atom in the Periodic Table; elements in the Periodic Table are ranked by Atomic Number – example hydrogen has 1, helium has 2, etc. There are only 100 elements. Why?

atomic weight.noun
a number representing the weight of one atom, as compared to the weight of another atom, Carbon-14, used as the standard

atone.verb
inflected form(s)-atoned, atoning
transitive senses-reconcile; to supply satisfaction for; expiate
intransitive senses-to make amends 
atonement.noun
reconciliation; the reconciliation of God and man through the sacrificial death of Jesus Christ; reparation for an offense or injury; satisfaction

ATP.noun
an adenosine derived nucleotide, C₁₀H₁₆N₅O₁₃P₃, (10 parts {molecules} carbon, 16 parts hydrogen, 5 parts nitrogen, 13 parts oxygen, 3 parts phosphorus) that supplies large amounts of energy to cells for various biochemical processes, including muscle contraction and sugar metabolism, through its hydrolysis to ADP (adenosine triphosphate)

attain, attained, attaining, attains.transitive verbs
transitive use.to reach or come to; to arrive at (he attained the age of ninety); accomplish; achieve; to gain
intransitive use.to succeed in a directed effort, process, or progression (eventually attained to wisdom; attained to high office)
attainable.adjective
that can be attained 
attainability, attainableness.nouns
the quality of being attainable 
attainment.noun
anything attained

attenuate.transitive verb
reduce in force; make thin; rarefy

attest, attested, attesting, attests.transitive verbs
to declare to be true or genuine and correct; to certify by signature or oath; indicate--
attestation.adjective
intransitive use-to bear witness; give testimony (attested to their good faith) attestant, attestation, attester or attestor.noun

attitude.noun
a state of mind or a feeling; disposition; a mental position with regard to a fact or state (his attitude toward the loss was as his father would have expected him to have, one of appreciation for his father being around him so long, and not one of grieving over the passing of his father)
attitudinal.adjective

attribute, attributed, attributing, attributes.transitive verbs
to relate to a particular cause or source; ascribe; to regard as the work of a specified agent or creator (attributed the painting to Titian {short for Tiziano Vecellio, 1488?-1576; an Italian painter who introduced vigorous colors and the compositional use of backgrounds})

attribute.noun
an inherent.characteristic (she has the attributes {love, kindness, persistence} of her mother); a quality or characteristic inherent in or ascribed to someone or something, a characteristic belonging to a specific person, thing, or office
synonyms-imputed, ascribed, assign, refer
attributable.adjective
attributer.or.attributor.noun
attribution.noun
attributer-or-attributor.nouns
the act of attributing, especially of establishing a particular person, place, or time as the creator, provenance, or era of a work of art; something, such as a quality or characteristic, that is related to a particular possessor; an attribute
attributional.adjective

audacity.noun
insolence, impudence
suggests either great presumption or defiance of social convention, morals, absolutes, etc. 
synonym-temerity
audacious.adjective
daring, reckless, bold; presumptuous; insolent, impudent