Refers to pile in various technical and everyday senses: fabric with raised surface (pile fabric), nuclear reactor pile, structural pile, etc. Context determines the specific meaning.
このカーペットはパイルが長くて柔らかい。
This carpet has a long, soft pile.
原子炉のパイルは厳重
The nuclear reactor pile is strictly controlled.
pile driver
Borrowed from English 'pile'.