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to unite formally, as by treaty, league, marriage, or the like (usually followed by with or to):
Russia allied itself to France.
to associate or connect by some mutual relationship, as resemblance or friendship.
to enter into an ; join; unite.
a person, group, or nation that is associated with another or others for some common cause or purpose:
Canada and the United States were allies in World War II.
Biology. a plant, animal, or other organism bearing an evolutionary relationship to another, often as a member of the same family:
The squash is an ally of the watermelon.
a person who associates or cooperates with another; supporter.
an adverbial suffix attached to certain adjectives with stems in -ic that have no forms ending in -ical: terrifically.
Contemporary Examples

He was saluting what his ally, Nelson Mandela, had done to lead his country into democracy.
Chris Matthews Remembers Nelson Mandela MSNBC December 12, 2013

The fundamental point is that Turkey remains a U.S. ally—and a more important one than ever.
Mideast’s New Superpower Owen Matthews September 20, 2011

At first the authorities refuse to admit him, but he finds an ally in a dedicated teacher played by Naomie Harris.
Will Brits Own the Oscars? Stephen Farber September 8, 2010

Religion has been both the antagonist and the ally of science.
How Noah’s Flood Spurred Science: David R. Montgomery’s ‘The Rocks Don’t Lie’ David Sessions August 27, 2012

The Kabul government, our ally, would find itself in the middle between two friends.
GOP Candidates Are Wrong to Urge a Second Front War in Iran Bruce Riedel January 26, 2012

Historical Examples

To McCalloway his thoughts had turned for the succour of a steadying calm—and that one ally was no longer in reach.
The Tempering Charles Neville Buck

In less than five minutes, she conquered old Nan, and secured her as an ally for ever.
Hetty’s Strange History Anonymous

He was Pharisaic ally thankful that he was not as that conglomerate group in the Bannister box.
The Trumpeter Swan Temple Bailey

Italy was at loggerheads with Austria, her ally, and about to break.
The Story of the Great War, Volume III (of VIII) Various

He must ally himself with no party and yet command the confidence of all parties.
Great Men and Famous Women. Vol. 3 of 8 Various

verb (əˈlaɪ) -lies, -lying, -lied usually foll by to or with
to unite or be united, esp formally, as by treaty, confederation, or marriage
(transitive; usually passive) to connect or be related, as through being similar or compatible
noun (ˈælaɪ; əˈlaɪ) (pl) -lies
a country, person, or group allied with another
a plant, animal, substance, etc, closely related to another in characteristics or form
v.

late 13c., “to join in marriage,” from Old French alier “combine, unite,” from a differentiated stem of aliier (from Latin alligare “bind to;” see alloy). Meaning “to form an alliance, join, associate” is late 14c. Related: allied; allying.
n.

late 14c., “relative, kinsman,” from ally (v.); mid-15c. in the sense of “one united with another by treaty or league.”