active

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Being in physical motion.
• adjective Functioning or capable of functioning.
• adjective Marked by energetic activity; busy.
• adjective Involving or requiring physical exertion and energy.
• adjective Being in a state of action; not quiescent.
• adjective Erupting or liable to erupt; not dormant.
• adjective Marked by or involving direct participation.
• adjective Currently in use or effect.
• adjective Openly acknowledged or expressed.
• adjective Producing an intended action or effect.
• adjective Indicating that the grammatical subject of a verb is performing or causing the action expressed. Used of a verb form or voice.
• adjective Expressing action rather than a state of being. Used of verbs such as run, speak, and move.
• adjective Of or relating to the management of an investment portfolio by continually making investment decisions based on new information as opposed to maintaining a predetermined strategy or reproducing the returns of a market or index.
• adjective Being a source of electrical energy, as a generator.
• adjective Capable of converting or amplifying voltages or currents, as a diode or transistor.
• adjective Being on full military duty and receiving full pay.
• noun The active voice.
• noun A construction or form in the active voice.
• noun A participating member of an organization.

from The Century Dictionary.

• In psychology, representative of a type of character whose dominant characteristic is a natural and constantly renewed tendency to action.
• Having the power or property of acting; tending to cause change or communicate action or motion; capable of exerting influence: opposed to passive: as, attraction is an active power.
• Specifically In medicine, acting quickly; producing immediate effects: as, active remedies or treatment.
• Having the power of quick motion, or disposition to move with speed; nimble; lively; brisk; agile: as, an active animal.
• Busy; constantly engaged in action; acting with vigor and assiduity: opposed to dull, slow, or indolent: as, an active officer; also to sedentary: as, an active life.
• In a state of action; marked by movement or operation; in actual progress or motion; not quiescent, dormant, or suspended: as, to take active proceedings against an offender; to engage in active hostilities.
• Hence In com., marked by quickness or frequency; brisk; lively; coming or moving freely or abundantly: as, an active trade or demand for goods; active freights or stocks. Requiring action or exertion; practical; operative; producing real effects: opposed to speculative: as, the active duties of life; the active powers of the mind.
• In grammar, signifying the performance and not the endurance of an action: opposed to passive.
• Reid uses the term to denote the will, appetites, affections, etc.; but that use has been generally condemned.
• The state of having a place on the active list, under full pay: used in contradistinction to being on the retired list, under reduced pay.
• Active regards either mind or body; there is no sinister sense of the word. The activity may be merely for its own sake. Active is opposed to lazy, inert, or quiescent: an active mind, life, person.
• Busy is active about something that is supposed to be useful. As applied to disposition, the word has acquired a bad sense, that of meddlesome: a busybody; he is too busy about others' affairs. An officious person is one whose efforts to be active or busy for others' benefit come, through his lack of judgment, to be regarded as annoying or intrusive. See impertinent.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Having the power or quality of acting; causing change; communicating action or motion; acting; -- opposed to passive, that receives; ; the powers of the mind.
• adjective Quick in physical movement; of an agile and vigorous body; nimble.
• adjective In action; actually proceeding; working; in force; -- opposed to quiescent, dormant, or extinct
• adjective Given to action; constantly engaged in action; energetic; diligent; busy; -- opposed to dull, sluggish, indolent, or inert
• adjective Requiring or implying action or exertion; -- opposed to sedentary or to tranquil
• adjective Given to action rather than contemplation; practical; operative; -- opposed to speculative or theoretical.
• adjective Brisk; lively.
• adjective Implying or producing rapid action
• adjective Applied to a form of the verb; -- opposed to passive. See Active voice, under Voice.
• adjective Applied to verbs which assert that the subject acts upon or affects something else; transitive.
• adjective Applied to all verbs that express action as distinct from mere existence or state.
• adjective money, or property that may readily be converted into money.