between

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

• preposition In or through the position or interval separating.
• preposition Intermediate to, as in quantity, amount, or degree.
• preposition Connecting spatially.
• preposition Usage Problem Associating or uniting in a reciprocal action or relationship.
• preposition In confidence restricted to.
• preposition By the combined effort or effect of.
• preposition In the combined ownership of.
• preposition As measured against. Often used to express a reciprocal relationship.
• adverb In an intermediate space, position, or time; in the interim.
• idiom (in between) In an intermediate situation.
• idiom (in between times) During an intervening period; in the meantime.

from The Century Dictionary.

• noun One of a grade of needles between sharps and blunts.
• In the space which separates (two points, places, objects, or lines); at any point of the distance from one to the other of: as, between the eyes; between Washington and Philadelphia; the prisoner was placed between two policemen.
• In intermediate relation to, as regards time, quantity, or degree: as, it occurred between his incoming and outgoing; a baronet is between a knight and a baron; they cost between $5 and $6 each; between 12 and 1 o'clock.
• In the mutual relations of: as, discord exists between the two families.
• From one to another of, as in the exchange of actions or intercourse.
• In the joint interest or possession of: as, they own the property between them.
• By the action, power, or effort of one or both of.
• In regard to the respective natures or qualities of: as, to distinguish between right and wrong.
• In regard to one or the other of: as, to choose between two things.
• In the intermediate space; in intermediate relation as regards time, etc.: with an object understood.

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

• noun Poetic & R. Intermediate time or space; interval.
• preposition In the space which separates; betwixt.
• preposition Used in expressing motion from one body or place to another; from one to another of two.
• preposition Belonging in common to two; shared by both.
• preposition Belonging to, or participated in by, two, and involving reciprocal action or affecting their mutual relation.
• preposition With relation to two, as involved in an act or attribute of which another is the agent or subject.
• preposition In intermediate relation to, in respect to time, quantity, or degree.
• preposition the space, or in the space, between the decks of a vessel.
• preposition in confidence; with the understanding that the matter is not to be communicated to others.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• preposition In the position or interval that separates (two things), or intermediate in quantity or degree. (See the Usage notes below.)
• preposition Done together or reciprocally.
• preposition Shared in confidence.
• preposition In transit from (one to the other, or connecting places).
• preposition Combined (by effort or ownership).
• preposition One of (representing a choice).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adverb in between
• adverb in the interval