Generaly Lightweight software means that it has small size and system resource requirements.
But there are more criteria that could be applied to this term. Let's take a look at them:
Low system resource requirements
First of all lightweight software should have low system resource requirements. Of cource some kinds of software like graphic editors could require lots or RAM, but if a program utilizes significaly less memory than other programs with same features it could be considered as lightweight.
Small distributive size
It is obviously that it is better when distributive size is small. User needs less time for program to download and less space to store it. But some programs could come with large clipart and media libraries. Even so program could be considered as lightweigh if it has several distributive with different sizes so user could choose what fits his needs best.
Easy-to-install
Lightweight program should install easily. It should ask user as minimum questions as possible but could allow advanced setup for an expert. Silent install option for batch installing will be a big plus.
Easy-to-use
User interface of lightweight software should be intuitive and easy-to-use so user could find required options fast and refer to documentation as rare as possible.
