"Use"

Using a card refers to choosing to trigger any ability in that card's text, including some conditional abilities. Required conditional abilities are forced to be triggered and are thus not used, but Optional conditional abilities are used. Using can apply broadly to card types as referenced by abilities (e.g. "using an icebreaker" means using any chosen abilities on an icebreaker card).

Page 3, Column 1, Paragraph(s) 4, FAQ
Any time an ability is optional, the player resolving the ability is "using" the ability. This includes all paid abilities and all optional conditional abilities (triggered abilities that use the word "may" or "allows").

FAQ 1.1.28[]

From the FAQ. Also numbers 1.2.6, 1.3.6, 1.4.21, and 1.5.20

Page 10, Column 1, Paragraph(s) 8, FAQ
Can the Runner use the recurring credit on Cyberfeeder to pay for using Femme Fatale's bypass ability or Darwin's gaining counters ability?
Yes. Triggering any ability on an icebreaker is considered to be using it.