What does the word Denier mean? Why is it mentioned so much with outdoor and adventure gear?

The term denier is a unit of measure in textiles for a linear mass of a fibre.  It is the mass in grams for 9,000 metres of the fibre. So 9,000 metres of a 450 denier thread weighs 450 grams. A single strand of silk is is approximately one denier; meaning a 9000 metre strand of silk weighs about one gram.

Heavier denier fabrics are typically used in outdoor gear that needs to withstand lots of wear and tear and be durable.  600 up to 1680 denier nylon fabric is typically used in luggage in heavy wear areas to prevent pinholes and tearing occurring in the fabric. In outerwear, heavier trekking jackets usually have higher denier material in wear areas for where a pack contacts the jacket on the chest, shoulders and the back. 

Higher denier fabrics do not necessarily outperform lower denier material. For example, a 420 denier nylon fabric is actually stronger than a 600 denier polyester fabric. Nylon fabric has a better abrasion resistance than polyester or cotton. So it is common to see nylon as the fabric choice in backpacks, luggage and outwear in heavy wear zones.

Ultralight backpacks, tents, mats, sleeping bags and jackets use lower denier fabrics to keep the weight of the item down - for example, 10 denier fabric tent flys, 15 denier fabric tent floors, 12 denier jacket face fabrics, 20 denier sleeping mats, 30 denier cordura nylons in packs all keep the weight of this gear down but will require some extra TLC to ensure this gear does not tear or rip when in use.