(i-SPAL-yuhr, -yay) 
noun
A tree trained to grow flat against a wall.
verb tr.
To train a tree in such a way.
Etymology
From French espalier, from Italian spalliera (shoulder support), from spalla (shoulder), from Latin spatha (broadsword)
The word originally referred to the trellis or framework on which a tree was trained to grow in a plane. Typically fruit trees such as apple and pear are grown as espaliers. The advantages of an espalier are that it can be grown in a small space, gets more sunlight, and provides easier access to the fruit.
A picture of a ginkgo espalier:

[Image credits: Chris Heiler www.FountainheadGardens.com
Usage
"First, you get to see the Food Garden with all its first-class espaliered fruit trees trained into the shape of vases and goblets, as well as the classic Belgian fence." — Steve Whysall; Tale of Two Gardens (University of B.C. Botanical Garden and the Nitobe Memorial Garden); Vancouver Sun (Canada); Aug 29, 2008.