WordNet sense 1 (difficulty that causes worry or emotional tension):

stress

WordNet sense 2 ((psychology) nervousness resulting from mental stress):

mental strain, nervous strain

WordNet sense 3 ((biology) a group of organisms within a species that differ in trivial ways from similar groups):

form, variant, var.

WordNet sense 4 (a special variety of domesticated animals within a species):

breed, stock

WordNet sense 5 (a succession of notes forming a distinctive sequence):

air, line, melody, tune, melodic line, melodic phrase

WordNet sense 6 (the general meaning or substance of an utterance):

tenor

WordNet sense 7 (test the limits of):

try

WordNet sense 8 (become stretched or tense or taught):

tense

WordNet sense 9 (separate by passing through a sieve or other straining device to separate out coarser elements):

sieve, sift

WordNet sense 10 (remove by passing through a filter):

filter, filtrate, filter out, separate out

WordNet sense 11 (rub through a strainer or process in an electric blender):

puree

WordNet sense 12 (use to the utmost; exert vigorously or to full capacity):

extend

WordNet sense 13 (to exert much effort or energy):

reach, strive

WordNet sense 14 (an effortful attempt to attain a goal):

pains, nisus, striving

WordNet sense 15 (an intense or violent exertion):

straining

WordNet sense 16 (the act of singing):

song

WordNet sense 17 (alter the shape of (something) by stress):

deform, distort

WordNet sense 18 (cause to be tense and uneasy or nervous or anxious):

tense up