Proxy

In general, a "proxy" is a substitute or go-between. For example, a proxy server takes web page requests from a computer and transmits in on their behalf, and then passes the resulting page back.

In wargames, "proxy" is used as a verb: to "proxy" a model means to use some other model to represent it. For example, Jokaero look rather silly, but AT-43 Karmans look way cool, and they're made to the same scale, so people can and do use them as substitutes.

The degree to which this practice is accepted depends on the company and setting. Battlefront Miniatures (makers of Flames of War) is pretty chill about proxies, while Games Workshop forbids them from their official tournaments and stores.

The skill of the conversion can make or break the acceptance: a Land Raider masterfully built out of Lego bricks will likely be accepted (and maybe even complimented), while a Land Raider represented by a juice box probably won't.