Thickener, composition, and sheet

A thickener comprising cellulose fibers having a fiber width of 8 nm or less and water, wherein

• the thickener is a slurry or a gel,
• a diluted solution of the thickener having a solids concentration of 0.5% by mass has a shear viscosity of 3000 mPa·s or more at 25° C. and at a shear velocity of 1 s−1,
• a polymerization degree of the cellulose fibers having a fiber width of 8 nm or less is 280 or more, and
• when the thickener is filled in a colorless and transparent glass cell having an inside dimension of 1 cm in depth×4 cm in width×4.5 cm in height and the thickener is then irradiated with ultraviolet rays with a wavelength of 300 nm or more and 400 nm or less, using a xenon lamp, from the side of the maximum area surface of the glass cell, so as to be an irradiance of 180 W/m2 and an integrated light amount of 500 mJ/m2,
• the amount of a change in the yellowness before and after ultraviolet irradiation measured in accordance with JIS K 7373 is 10 or less.