Whatever your definition, a weed is simply a plant growing out of place. Generally, but not always, weeds are hard to eliminate. There are several ways to control them. First, consider prevention. Removing undesirable plants before they flower and develop seeds will reduce the number of weed seeds added to your garden.
Most weeds produce a huge number of seeds.
When adding mulch or compost to your garden, be sure that it is weed free. Old weedy hay may be a cheap source of mulch, but it could add unwanted weed seeds that you would have to deal with in future years. While hot composting may kill many seeds, cold composting may not. Remember too, that many seeds lie dormant in the soil for years, so prevention will not guarantee a weed free yard. Secondly, consider control methods.