1) Enervation 2) Enfeeblement 3) Exhaustion 4) French word used in English 5) Weakening 1) Enfeeblement 
• (n.) The act or process of debilitating, or the condition of one who is debilitated; weakness.
De·bil`i·ta'tion noun [ Latin debilitatio : confer French débilitation .] The act or process of debilitating, or the condition of one who is debilitated; weakness. 
[n] - serious weakening and loss of energy
enervation noun serious weakening and loss of energy
In general this term refers to anything that is getting weaker with time.