attrition

Short definition from Wiktionary

Noun

1. 1
   Grinding down or wearing away by friction.
2. 2
   Grinding down or wearing away by friction: The wearing of teeth due to their grinding.
3. 3
   A gradual reduction in number.
4. 4
   A gradual reduction in number: A gradual, natural reduction in membership or personnel, as through injury, incapacitation, retirement, resignation, or death.
5. 5
   A gradual reduction in number: The loss of participants during an experiment.
6. 6
   A gradual reduction in number: The loss of a first or second language or a portion of that language.
7. 7
   Imperfect contrition or remorse.

Verb

1. 1
   To grind or wear down through friction.
2. 2
   To reduce the number of (jobs or workers) by not hiring new employees to fill positions that become vacant (often with out).
3. 3
   To undergo a reduction in number.